Add more sections to the CAPI docs (#655)

* Add more sections to the CAPI docs

* Address comments

docs/src/capi/explanation/capi-ck8s.md

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+# Cluster API - {{product}}
+
+ClusterAPI (CAPI) is an open-source Kubernetes project that provides a declarative API for cluster creation, configuration, and management. It is designed to automate the creation and management of Kubernetes clusters in various environments, including on-premises data centers, public clouds, and edge devices.
+
+CAPI abstracts away the details of infrastructure provisioning, networking, and other low-level tasks, allowing users to define their desired cluster configuration using simple YAML manifests. This makes it easier to create and manage clusters in a repeatable and consistent manner, regardless of the underlying infrastructure. In this way a wide range of infrastructure providers has been made available, including but not limited to Amazon Web Services (AWS), Microsoft Azure, Google Cloud Platform (GCP), and OpenStack.
+
+CAPI also abstracts the provisioning and management of Kubernetes clusters allowing for a variety of Kubernetes distributions to be delivered in all of the supported infrastructure providers. {{product}} is one such Kubernetes distribution that seamlessly integrates with Cluster API.
+
+With {{product}} CAPI you can:
+- provision a cluster with:
+    - Kubernetes version 1.31 onwards
+    - risk level of the track you want to follow (stable, candidate, beta, edge)
+    - deploy behind proxies
+- upgrade clusters with no downtime:
+    - rolling upgrades for HA clusters and worker nodes
+    - in-place upgrades for non-HA control planes and worker nodes
+
+Please refer to the “Tutorial” section for concrete examples on CAPI deployments:
+
+
+## CAPI architecture
+
+Being a cloud-native framework, CAPI implements all its components as controllers that run within a Kubernetes cluster. There is a separate controller, called a ‘provider’, for each supported infrastructure substrate. The infrastructure providers are responsible for provisioning physical or virtual nodes and setting up networking elements such as load balancers and virtual networks. In a similar way, each Kubernetes distribution that integrates with ClusterAPI is managed by two providers: the control plane provider and the bootstrap provider. The bootstrap provider is responsible for delivering and managing Kubernetes on the nodes, while the control plane provider handles the control plane’s specific lifecycle.
+
+The CAPI providers operate within a Kubernetes cluster known as the management cluster. The administrator is responsible for selecting the desired combination of infrastructure and Kubernetes distribution by instantiating the respective infrastructure, bootstrap, and control plane providers on the management cluster.
+
+The management cluster functions as the control plane for the ClusterAPI operator, which is responsible for provisioning and managing the infrastructure resources necessary for creating and managing additional Kubernetes clusters. It is important to note that the management cluster is not intended to support any other workload, as the workloads are expected to run on the provisioned clusters. As a result, the provisioned clusters are referred to as workload clusters.
+
+Typically, the management cluster runs in a separate environment from the clusters it manages, such as a public cloud or an on-premises data center. It serves as a centralized location for managing the configuration, policies, and security of multiple managed clusters. By leveraging the management cluster, users can easily create and manage a fleet of Kubernetes clusters in a consistent and repeatable manner.
+
+The {{product}} team maintains the two providers required for integrating with CAPI:
+
+- The Cluster API Bootstrap Provider {{product}} (**CABPCK**) responsible for provisioning the nodes in the cluster and preparing them to be joined to the Kubernetes control plane. When you use the CABPCK you define a Kubernetes Cluster object that describes the desired state of the new cluster and includes the number and type of nodes in the cluster, as well as any additional configuration settings. The Bootstrap Provider then creates the necessary resources in the Kubernetes API server to bring the cluster up to the desired state. Under the hood, the Bootstrap Provider uses cloud-init to configure the nodes in the cluster. This includes setting up SSH keys, configuring the network, and installing necessary software packages.
+
+- The Cluster API Control Plane Provider {{product}} (**CACPCK**) enables the creation and management of Kubernetes control planes using {{product}} as the underlying Kubernetes distribution. Its main tasks are to update the machine state and to generate the kubeconfig file used for accessing the cluster. The kubeconfig file is stored as a secret which the user can then retrieve using the `clusterctl` command.
+
+```{figure} ./capi-ck8s.svg
+   :width: 100%
+   :alt: Deployment of components
+
+   Deployment of components
+```

docs/src/capi/explanation/index.md

@@ -15,6 +15,7 @@ Overview <self>
 
 about
 security
+capi-ck8s.md
 
 ```
 

docs/src/capi/howto/custom-ck8s.md

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+# Install custom {{product}} on machines
+
+By default, the `version` field in the machine specifications will determine which {{product}} is downloaded from the `stable` rist level. While you can install different versions of the `stable` risk level by changing the `version` field, extra steps should be taken if you're willing to install a specific risk level.
+This guide walks you through the process of installing custom {{product}} on workload cluster machines.
+
+## Prerequisites
+
+To follow this guide, you will need:
+
+- A Kubernetes management cluster with Cluster API and providers installed and configured.
+- A generated cluster spec manifest
+
+Please refer to the [getting-started guide][getting-started] for further
+details on the required setup.
+
+In this guide we call the generated cluster spec manifrst `cluster.yaml`.
+
+## Overwrite the existing `install.sh` script
+
+The installation of the {{product}} snap is done via running the `install.sh` script in the cloud-init.
+While this file is automatically placed in every workload cluster machine which hard-coded content by {{product}} providers, you can overwrite this file to make sure your desired content is available in the script. 
+
+As an example, let's overwrite the `install.sh` for our control plane nodes. Inside the `cluster.yaml`, add the new file content:
+```yaml
+apiVersion: controlplane.cluster.x-k8s.io/v1beta2
+kind: CK8sControlPlane
+...
+spec:
+  ...
+  spec:
+    files:
+    - content: |
+        #!/bin/bash -xe
+        snap install k8s --classic --channel=latest/edge
+      owner: root:root
+      path: /capi/scripts/install.sh
+      permissions: "0500"
+```
+
+Now the new control plane nodes that are created using this manifest will have the `latest/edge` {{product}} snap installed on them!
+
+## Use `preRunCommands`
+
+As mentioned above, the `install.sh` script is responsible for installing {{product}} snap on machines. `preRunCommands` are executed before `install.sh`. You can also add an install command to the `preRunCommands` in order to install your desired {{product}} version. 
+
+```{note}
+Installing the {{product}} snap via the `preRunCommands`, does not prevent the `install.sh` script from running. Instead, the installation process in the `install.sh` will fail with a message indicating that `k8s` is already installed.
+This is not considered a standard way and overwriting the `install.sh` script is recommended.
+```
+
+Edit the `cluster.yaml` to add the installation command:
+```yaml
+apiVersion: controlplane.cluster.x-k8s.io/v1beta2
+kind: CK8sControlPlane
+...
+spec:
+  ...
+  spec:
+    preRunCommands:
+    - snap install k8s --classic --channel=latest/edge
+```
+
+<!-- LINKS -->
+[getting-started]: ../tutorial/getting-started.md

docs/src/capi/howto/index.md

@@ -16,6 +16,9 @@ Overview <self>
 
 external-etcd
 rollout-upgrades
+upgrade-providers
+migrate-management
+custom-ck8s
 ```
 
 ---

docs/src/capi/howto/migrate-management.md

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+# Migrate the managment cluster
+
+Management cluster migration is a really powerful operation in the cluster’s lifecycle as it allows admins
+to move the management cluster in a more reliable substrate or perform maintenance tasks without disruptions.
+In this guide we will walk through the migration of a management cluster.
+
+## Prerequisites
+
+In the [Cluster provisioning with CAPI and {{product}} tutorial] we showed how to provision a workloads cluster. Here, we start from the point where the workloads cluster is available and we will migrate the management cluster to the one cluster we just provisioned.
+
+## Install the same set of providers to the provisioned cluster
+
+Before migrating a cluster, we must make sure that both the target and source management clusters run the same version of providers (infrastructure, bootstrap, control plane). To do so, `clusterctl init` should be called against the target cluster:
+
+```
+clusterctl get kubeconfig <provisioned-cluster> > targetconfig
+clusterctl init --kubeconfig=$PWD/targetconfig --bootstrap ck8s --control-plane ck8s --infrastructure <infra-provider-of-choice>
+```
+
+## Move the cluster
+
+Simply call:
+
+```
+clusterctl move --to-kubeconfig=$PWD/targetconfig
+```
+
+<!-- LINKS -->
+[Cluster provisioning with CAPI and {{product}} tutorial]: ../tutorial/getting-started.md

docs/src/capi/howto/upgrade-providers.md

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+# Upgrading the providers of a management cluster
+
+In this guide we will go through the process of upgrading providers of a management cluster.
+
+## Prerequisites
+
+We assume we already have a management cluster and the infrastructure provider configured as described in the [Cluster provisioning with CAPI and {{product}} tutorial]. The selected infrastructure provider is AWS. We have not yet called `clusterctl init` to initialise the cluster.
+
+## Initialise the cluster
+
+To demonstrate the steps of upgrading the management cluster, we will begin by initialising a desired version of the {{product}} CAPI providers.
+
+To set the version of the providers to be installed we use the following notation:
+
+```
+clusterctl init --bootstrap ck8s:v0.1.2 --control-plane ck8s:v0.1.2 --infrastructure <infra-provider-of-choice>
+```
+
+## Check for updates
+
+With `clusterctl` we can check if there are any new versions of the running providers:
+
+```
+clusterctl upgrade plan
+```
+
+The output shows the existing version of each provider as well as the version that we can upgrade into:
+
+```text
+NAME                 NAMESPACE       TYPE                     CURRENT VERSION   NEXT VERSION
+bootstrap-ck8s       cabpck-system   BootstrapProvider        v0.1.2            v0.2.0
+control-plane-ck8s   cacpck-system   ControlPlaneProvider     v0.1.2            v0.2.0
+cluster-api          capi-system     CoreProvider             v1.8.1            Already up to date
+infrastructure-aws   capa-system     InfrastructureProvider   v2.6.1            Already up to date
+```
+
+## Trigger providers upgrade
+
+To apply the upgrade plan recommended by `clusterctl upgrade plan`, simply:
+
+```
+clusterctl upgrade apply --contract v1beta1
+```
+
+To upgrade each provider one by one, issue:
+
+```
+clusterctl upgrade apply --bootstrap cabpck-system/ck8s:v0.2.0
+clusterctl upgrade apply --control-plane cacpck-system/ck8s:v0.2.0
+```
+
+<!-- LINKS -->
+[Cluster provisioning with CAPI and {{product}} tutorial]: ../tutorial/getting-started.md