Reorganize codeowners and move some file

1. Updates the handbook group to the content-sites group.
2. Cleanup of codeowners file, and re-organizes it:
   1. Remove team members who are no longer at GitLab
   2. Sorts sections alphabetically except handbook related ones, and controlled documents, which are at the bottom.
   3. Updates the comment at the top to clarify, and remove redundant comments.
   4. Sorts list of pages/directories in a section alphabetically.
   5. Combine some sections that were duplicated, or only had 1-2 lines.
   6. Ensures all top-level pages and directories are listed.
3. No changes to codeowners themselves, except adds e-group to handbook stuff, and makes support directory optional (leaves specific pages required).
4. A few pages were moved to directories where it made sense. Redirects (aliases) added in these cases.
5. A few pages were deleted where obsolete or empty.

Related to https://gitlab.com/gitlab-com/content-sites/handbook/-/issues/173