Create new CODEOWNERS file

What does this MR do?

NOTE: @gitlab-org/maintainers/gitlab-pages doesn't yet exist. https://gitlab.com/gitlab-com/team-member-epics/access-requests/-/issues/29877+ is the AR to have this group created.

This MR creates a new .gitlab/CODEOWNERS file using @gitlab-org/maintainers/gitlab-pages as those responsible for *.

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines