Adding documentation and example for runner sub-chart

What does this MR do?

Updated documentation and added example for gitlab-runner config. This was released in v13.6, but was quite an investigation to figure out how to use it. I've tested on my production version (v13.10)

Change is proposed to this document https://docs.gitlab.com/charts/charts/gitlab/gitlab-runner/

Currently relevant information was found only in closed MRs and on https://gitlab.com/gitlab-org/charts/gitlab-runner/-/blob/master/values.yaml