Remove "topics" from CE/EE docs landing page, reorganize the subjects as topics
What
Reorg https://docs.gitlab.com/ce/ and https://docs.gitlab.com/ee/ in topics, not in user- and admin-related docs.
We want to make the idea (https://gitlab.com/gitlab-org/gitlab-ee/issues/1912) of the "topics" pages the way we organize our docs' landing page. Instead of having topic- and user- and admin- docs, we will have topic- and admin-only- docs.
- The first bunch of links go to: topic page (when exists), or user-doc main page
- The 2nd bunch of links go to admin-only related stuff (example: installing GitLab)
Topics examples:
- Pages: https://docs.gitlab.com/ce/user/project/pages/
- CI: https://docs.gitlab.com/ce/ci/
- Auth: https://docs.gitlab.com/ce/topics/authentication/
Restructure
We don't need an immediate change in the file tree, we need just to reorg the landing page.
cc/ @seanpackham @axil