Administrator documentation for Pages access control

What does this MR do?

Documents Pages access control from the admin's point of view.

What are the relevant issue numbers?

• https://gitlab.com/gitlab-org/gitlab-ce/issues/33422
• gitlab-pages!94 (merged)
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18589

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [-] Tests added for this feature/bug
• [-] Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides