Document the refs that GitLab uses

From gitlab-foss#44431 (comment 64122437)

By the way it looks like we don't have documentation about what are the refs that GitLab uses and I think it could be very useful. It's at least the third time that I have been searching for documentation about this without finding anything except some issues.

gitlab-foss#36803 (moved) for example contains some info that would belong to the documentation where it could be improved upon.

From gitlab-foss#36803 (moved)

• Branches (refs/heads/**/*)
• Tags (refs/tags/**/*)
• Merge requests (refs/merge-requests/[MERGE_REQUEST_IID]/head) (it's possible to replace it with refs/keep-around, but this is also a feature)
• Commits which have notes (refs/keep-around/[SHA])
• Commits which have pipelines/jobs (refs/keep-around/[SHA])
• Commits for deployments (refs/environments/[NAME]) (it's possible to replace it with refs/keep-around)