Create a first draft pixi migration guide

docs/pixi-migration.md

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+# pixi migration guide
+
+So you want to try pixi, the new project-centric Conda environment manager?
+We will walk you through the steps to get started, whether or not you are a conda-lock user.
+
+Perhaps "migration" is the wrong word, because pixi is an option, not a replacement.
+It will not override existing workflows.
+Removing pixi from your project is as simple as deleting the files indicated in step 2 below.
+
+## The steps to get started
+
+1. Visit <https://pixi.sh> and install pixi if you haven't already.
+    You can also find information about IDE integrations and other documentation.
+    *Note*: As of writing, VS Code's Python extension has native support for pixi.
+
+2. Prepare for the new files and directories related to pixi.
+
+    - `pixi.toml` defines the environment specifications and config options. We will generate this with `conda-lock`.
+
+    - `pixi.lock` is managed by pixi and maintains a particular solution to the environment specifications from `pixi.toml`.
+
+    - `.pixi/` is the environment cache directory where the environments from the lock file `pixi.lock` are installed and updated.
+        Pixi will recreate this directory if it is deleted.
+
+    Both `pixi.toml` and `pixi.lock` should be version controlled. The `.pixi/` directory should be ignored by `.gitignore`, `.dockerignore`, and/or similar.
+
+    *Note*: For Python projects it is possible to use `pyproject.toml` instead of `pixi.toml`.
+    For starters we recommend sticking with `pixi.toml` since as of writing there is very little advantage to using `pyproject.toml`.
+    Using separate files also provides more clear separation between Python project dependency management (`pyproject.toml`) and environment management (`pixi.toml`). Besides, migrating to `pyproject.toml` is mostly a matter of prefixing the table names in `pixi.toml` with `tool.pixi.`.
+
+3. Install the latest version of `conda-lock` if you don't already have it.
+
+    ```sh
+    pipx install --force "conda-lock>=3"
+    ```
+
+    or
+
+    ```sh
+    pipx upgrade conda-lock
+    ```
+
+4. Generate a `pixi.toml` file with `conda-lock`.
+
+    ```sh
+    conda-lock render-lock-spec --kind=pixi.toml --stdout \
+        --file=source-file-1 \
+        --file=source-file-2 \
+        ...
+        --pixi-project-name=my-project-name
+    ```
+
+    Each `--file` argument should include a path to some `environment.yml` or `pyproject.toml`.
+
+    The `--pixi-project-name` allows you to specify the name of the project.
+
+    For Python development, you can optionally include one or more `--editable python-project-name=relative-path` arguments to specify local packages. For example,
+
+    ```sh
+    --editable python-package=.
+    ```
+
+    works like `pip install -e .`. (More generally, `.` can be replaced with the relative path to a Python project root containing a `pyproject.toml` or `setup.py` file.)
+
+    For example for `conda-lock` we use the following command to generate our `pixi.toml` file:
+
+    ```sh
+    conda-lock render-lock-spec --kind=pixi.toml --stdout \
+      --file=environments/dev-environment.yaml \
+      --file=pyproject.toml \
+      --pixi-project-name=conda-lock \
+      --editable conda-lock=.
+    ```
+
+    When you are satisfied with the output, append `> pixi.toml` to the command to save it to a file.
+
+    Pay attention to any warnings that `conda-lock` may print. Some specifications may not be translatable, and others might not yet be implemented.
+
+    By default, dependencies from `pyproject.toml` will be converted to conda-forge dependencies. If you wish to keep them as PyPI dependencies, you can add the following table to the `pyproject.toml`:
+
+    ```toml
+    [tool.conda-lock]
+    default-non-conda-source = "pip"
+    ```
+
+    If you have a `test-dependencies.yml` file and want it to define a feature named `test`, then you should add
+
+    ```yaml
+    category: test
+    ```
+
+    to the top level of `test-dependencies.yml`.
+
+5. (Optional) Generate a `pixi.lock` file with pixi.
+
+    ```sh
+    pixi update
+    ```
+
+    This step is optional because pixi will automatically generate a lock file as needed from the other commands.
+    By running `pixi update` we are doing one step at a time.
+
+6. (Optional) Install the environment from the lock file.
+
+    ```sh
+    pixi install
+    ```
+
+    Optional because pixi automatically installs environments as needed.
+
+7. Start an activated shell to test the environment.
+
+    ```sh
+    pixi shell
+    ```
+
+    This command replaces `conda activate`. However, it uses a subshell instead of the current shell, so instead of `conda deactivate` you use `exit`.
+
+8. Commit `pixi.toml` and `pixi.lock` to your version control system.
+
+## Concepts
+
+### Features
+
+Conda-lock has a concept called "categories" for grouping dependencies.
+Each dependency belongs to one or more categories.
+When conda-lock generates a lock file, it solves for an environment containing all categories and determines which dependencies are needed for each category.
+An environment can be created by seleting some subset of categories, and only the necessary dependencies will be installed.
+
+Pixi has a similar but more clever concept called "features".
+Unlike categories, features are groups of dependency specifications that need not be mutually compatible.
+Named environments are specified by selecting some mutually compatible features.
+Then each named environment is solved for independently.
+
+There is also a default environment, and it can be overridden.
+We override the default environment to include all categories.
+
+...
+
+#### The default feature
+
+Explain the implicit default feature, and how it is used by `pixi.toml` tables that do not specify a feature.
+
+Also explain the default environment.
+
+All environments implicitly include the default feature!
+
+## Infrastructure
+
+### GitHub Actions
+
+If you used [setup-micromamba](https://github.com/mamba-org/setup-micromamba), [setup-miniconda](https://github.com/conda-incubator/setup-miniconda) or similar, then for pixi you can use [setup-pixi](https://pixi.sh/latest/advanced/github_actions/).
+
+For periodically updating the lock file, you can use [pixi-diff-to-markdown](https://pixi.sh/latest/advanced/updates_github_actions/).
+
+### Docker
+
+See [pixi-docker](https://github.com/prefix-dev/pixi-docker).