Docs: Add partner docs directory; Add module management doc (#8801)

docs/partners/managing-modules.md

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+# Customizing Jetpack's Modules
+
+Occasionally, hosts have asked how they could customize Jetpack's modules. The documentation below will provide use cases and instructions on how to manage modules.
+
+### How to get a list of modules
+
+Before we can disable or force a module on or off, we need to know the slug of the module we want to modify. To get the slugs, we have a couple of options:modules
+
+1) You can run `wp jetpack module list` to get list of module slugs as well as the status for each module.
+2) You can view the PHP files directly in the `modules` directory in Jetpack. Each PHP file that is in the top-level `modules` direcory is loaded as a module. The module slug is the file name minus the extension.
+
+### Disabling modules
+
+In cases where Jetpack has a competing feature with a host, it may be preferable to disable the module in Jetpack. This is easily done via a filter in Jetpack, however, we would caution hosts to consider user experience when making this decision. If a user expects a commonly used module to be available, and it's not, that could cause confusion and a support request for the host and/or Jetpack support.
+
+Now that we've got that warning out of the way, let's get to it!
+
+The filter that we'll use to disable a module is `jetpack_get_available_modules`. You can find documentation as well as an example for that filter here:PHP
+
+[https://developer.jetpack.com/hooks/jetpack_get_available_modules](https://developer.jetpack.com/hooks/jetpack_get_available_modules/)
+
+Here are a couple more alternative examples:
+
+#### Disabling the Photon module
+
+```php
+add_filter( 'jetpack_get_available_modules', 'jetpack_docs_filter_module_example' );
+function jetpack_docs_filter_module_example( $modules ) {
+	if( isset( $modules['photon'] ) ) {
+		unset( $modules['photon'] );
+	}
+
+	return $modules;
+}
+```
+
+#### Disabling the Lazy Images module
+
+```php
+add_filter( 'jetpack_get_available_modules', 'jetpack_docs_filter_module_example' );
+function jetpack_docs_filter_module_example( $modules ) {
+	return array_diff_key( $modules, array( 'photon' => 'Does not matter' ) );
+}
+```
+
+Note: As of Jetpack 5.8.0, there is a regression in filtering out the Photon module. While the module can be filtered out with the code above, the Jetpack admin UI shows an `undefined` string instead of hiding the setting. This behavior also applies to the Lazy Images module which was introduced in 5.8.0. This issue has been fixed in Automattic/jetpack#8780 which should go out in Jetpack 5.9.0.
+
+### Forcing modules to be active
+
+In some cases, a host may decide that forcing a module to be active may be preferable. For example, to minimize bandwidth on the host's server, a host could force the Photon module to be active. In these cases, it is possible to force a module on with the `jetpack_active_modules` filter.
+
+Documentation for that filter can be found at the following URL:preferable
+
+[https://developer.jetpack.com/hooks/jetpack_active_modules](https://developer.jetpack.com/hooks/jetpack_active_modules/)
+
+Here's an example of how to force the Photon module to be activated:preferable
+
+```php
+add_filter( 'option_jetpack_active_modules', 'jetpack_docs_filter_active_modules' );
+function jetpack_docs_filter_active_modules( $modules ) {
+	return array_values( array_merge( $modules, array( 'photon' ) ) );
+}
+```
+
+Note: The Jetpack admin UI isn't yet aware of this filter. So, if you force a filter to be active, the UI to enable/disable that module still displays. This issue is being tracked at this URL:
+
+[https://github.com/Automattic/jetpack/issues/8800](https://github.com/Automattic/jetpack/issues/8800)