Docs: Add docs for built-in project template settings
## Problem to solve
Docs are required for a new admin application setting and group-level cascading setting with the FF `built_in_project_templates_enabled`. This setting controls whether built-in project templates are available when creating new projects.
Affected areas:
- `doc/administration/settings/` (new page?)
- `doc/user/group/manage.md` (new section)
- `doc/api/settings.md` (new attributes)
- `doc/api/groups.md` (new attributes)
## Further details
The `built_in_project_templates_enabled` setting has two levels:
- Admin (instance level): Admins can enable or disable built-in project templates for the instance, and optionally enforce this setting for all groups using `lock_built_in_project_templates_enabled`.
- Group level: Groups can configure the setting independently under **Settings > General**, unless the admin has enforced it at the instance level.
> Cascading writes are not yet implemented. Changes at the instance level do not currently cascade to groups, and group changes do not cascade to subgroups. This is tracked in a follow-up work item: https://gitlab.com/gitlab-org/gitlab/-/work_items/593616.
The API documentation changes depend on the shape of the API, which is being determined in https://gitlab.com/gitlab-org/gitlab/-/issues/593619 and https://gitlab.com/gitlab-org/gitlab/-/issues/593620.
## Proposal
- Add `doc/administration/settings/built_in_project_templates.md` documenting the instance-level setting and the lock behavior.
- Add a new section to `doc/user/group/manage.md` documenting the group-level setting, including how it behaves when locked by an admin at the instance level, and the current limitation that changes do not yet cascade to subgroups.
- Update `doc/api/settings.md` and `doc/api/groups.md` with the new attributes.
## Other links/references
- Implementation MR (admin settings): https://gitlab.com/gitlab-org/gitlab/-/merge_requests/230641
- Implementation MR (group settings): https://gitlab.com/gitlab-org/gitlab/-/merge_requests/230790
- Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/593651
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