Add `JsonAttribute` to `gitlab.types` to handle JSON parsing for CLI
arguments.

Update `ProjectFeatureFlagManager` to use `JsonAttribute` for the
`strategies` attribute, replacing custom `create` and `update` overrides.

This adds support for managing user lists for project feature flags.
It introduces the `ProjectFeatureFlagUserList` object and manager,
and exposes it via `project.feature_flags_user_lists`.

New type `CommaSeparatedStringAttribute` is added to handle
comma-separated string values in API requests.

Add functional tests to ensure that feature flag strategies and scopes
can be removed using the `_destroy` flag in the update payload, as
supported by the GitLab API.

The `ProjectFeatureFlag` object uses `name` as its ID attribute.
Previously, modifying the `name` of a flag object and calling `save()`
would fail because the library would attempt to send a PUT request to
the *new* name, which does not yet exist, resulting in a 404 error.

This change overrides the `save()` method in `ProjectFeatureFlag` to
correctly handle renaming. It now detects when the `name` attribute has
been modified and uses the original name for the API request URL, while
sending the new name in the request body.

This makes renaming feature flags more intuitive and consistent with the
behavior of other objects in the library.

This adds the documentation for Project Feature Flags and Feature Flag
User Lists.

It includes:
- New documentation pages for Feature Flags and User Lists.
- Updates to the Project documentation to reference the new managers.

The existing documentation for `features.rst` covered the API for
managing GitLab's internal development feature flags, which requires
administrator access. This was easily confused with the newly added
project-level feature flags API.

To prevent ambiguity for users, this commit refactors the documentation by:
- Renaming `features.rst` to `gitlab_features.rst`.
- Updating the title and adding a note to clarify its specific,
  admin-only purpose.
- Adding a cross-reference to the new project-level feature flag
  documentation.

This adds unit tests for the new logic introduced with the feature
flags API.

- Verifies the custom `save()` method in `ProjectFeatureFlag` correctly
  handles renaming by using the old name in the URL and the new name
  in the request body.
- Verifies the `CommaSeparatedStringAttribute` correctly converts a
  Python list to a comma-separated string for API calls.
- Verifies the `JsonAttribute` correctly handles JSON string parsing.

The title overline in `docs/gl_objects/project_feature_flag_user_lists.rst`
was shorter than the title text, causing a Sphinx warning.