Create best practice documentation

Overview

There are some best practices to follow when using GitLab Runner, and we generally don't document this.

This came up with !1319 (merged) we fixed a regression where we changed how GitLab Runner cache the build directory since we now cache the root of the build directory for example /builds and not /builds/.... For #4180 (closed) we have an action point to raise awareness to the users that nothing should be inside of the build directory apart form their project (which GitLab Runner handles).

The first best practice that we need to document is: Nothing should be stored inside of the builds_dir, if the user mounts any helper scripts they need to be outside of the builds_dir. More detail on a real production issue user had because they didn't know of this #4174 (closed)