Handle a CODEOWNERS file in a .github-folder

When a repository has a .github/CODEOWNERS file we could mention to the user that they need to move the CODEOWNERS file to one of the locations we share with GitHub: the root of the repository, or docs/.

We might also try to use the .github/CODEOWNERS file, but it's not given that the usernames would be the same on both platforms. Users specified by email would keep working.

---

The following discussion from !6916 (merged) should be addressed:

• @reprazent started a discussion: (+1 comment)

  Should we also allow .github/CODEOWNERS to support people movingtogitlab?

  cc @jramsay