Add Route Map for runner docs
What does this MR do?
Adds a route map to this project, so the Review Apps link is more intelligent.
Just a slight modification from the one used in CE.
I've not done route maps before, so please check it carefully.
Why was this MR needed?
Adds a convenience for a docs reviewer we already have in the main CE/EE projects.
Are there points in the code the reviewer needs to double check?
Here are the docs for route maps: https://docs.gitlab.com/ee/ci/review_apps/index.html#route-maps.