Reorganize index sections for runner use cases
What does this MR do?
This reorganizes the sections in the runner documentation index to direct users more clear to differentiate between GitLab.com usage and own runner install and usage.
Why was this MR needed?
gitlab#396282 (closed) requires that we clarify installation instructions for runner, and the difference between installing the runner via the UI.
What's the best way to test this MR?
NA