Move admin content to administration folder

Proposal

We have files in a doc/user/admin_area directory.

These files are in the nav under Administer GitLab.

We should move the files to be with the admin docs, under doc/administration.

Steps to move

We'll be tracking the work in this spreadsheet.

1. Copy the file to the new location.
2. In the old file, add the redirect text.
3. Open a merge request with the changes.
4. Watch the pipeline for link failures.
5. Fix the failures in your branch and push another commit.
6. Watch the pipeline again and make sure it passes.
7. Assign to any of the following writers for review/merge: @phillipwells, @msedlakjakubowski, @dianalogan, @jglassman1, @lciutacu, @marcel.amirault, @rdickenson, @aqualls, @axil
8. Go to the gitlab-docs repo, to the navigation file and open an MR with the new location. Assign to the same writer for review/merge.
9. Update the spreadsheet to indicate that you're done with the file.

When we're done with all of this, we'll have to search the repo one last time for /user/admin_area and update any remaining links that we missed/that weren't caught by the pipeline. It's possible there are other UI links, and there will definitely be doc links.

Reasons I think it would be good to move

• We could tell administrators that all the content they need is at https://docs.gitlab.com/ee/administration. (Instead of also at https://docs.gitlab.com/ee/user/admin_area/)
• Right now, the top-level topics are strangely similarly named (GitLab Admin Area and Administrator documentation). We can fix this... There is a lot of work to be done, but getting the files in one place might be a good first step?
• For the content audit, it's a manual process to list the nav area where files live. So it would make updating the content audit easier.