Docs new topic "user/project/repository/index.md"
What does this MR do?
Adds repository documentation index to user/project/repository/index.md
Follows https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12155 (split in several MRs)
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/27653 and https://gitlab.com/gitlab-org/gitlab-ce/issues/28665
Edited by username-removed-236961