Update installation docs to include Docker, others
As per this comment https://gitlab.com/gitlab-com/support/issues/374#note_22056587, we're moving documentation around from the support workflow to the main installation docs
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
As per this comment https://gitlab.com/gitlab-com/support/issues/374#note_22056587, we're moving documentation around from the support workflow to the main installation docs