Document how to create group/project Kubernetes runners
Problem to solve
Discovered in gitlab-com/www-gitlab-com!31421 (comment 233057554)
Document how to use the GitLab to install Group and Project runners for Kubernetes. I was under the impression that documentation exists for doing this manually, but I couldn't actually find it at first glance.
There definitely doesn't seem to be any docs for doing this from the UI, which was added for groups in !16338 (merged), previously already available for projects (not documented, I think).