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.

CC @gl-docsteam.