Documentation topic: Authentication
See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html
What does this MR do?
Add a new topic to https://docs.gitlab.com/ce/topics/ = Authentication
Where: https://docs.gitlab.com/ce/topics/authentication/
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/30744
cc/ @axil FYI