Update documentation for GitLab Pages administration to include
a section about using API-based configuration.
Includes steps to disable this feature introduced via this [Pages MR](
https://gitlab.com/gitlab-org/gitlab-pages/-/merge_requests/264)
and [Omnibus MR]
(https://gitlab.com/gitlab-org/omnibus-gitlab/-/merge_requests/4057)

Add sync secrets section to pages admin