Move registering Runners info in a separate document
What does this MR do?
Split registering a Runner in its own document for modularity.
Why was this MR needed?
To reference the Runner registration in other places like https://docs.gitlab.com/ce/ci/runners/ where we mix commands from the Runner with user docs.
Cross project MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11948
Edited by Achilleas Pipinellis