WIP: add proxy documentation

What does this MR do?

This MR adds the documentation on how to run the runner behind a proxy.

/cc @axil First attempt for adding this to the docs. Is this the right place?
I added it as a seperate file so we can link to it from linux-repository.md and linux-manual.md
I'm pretty sure this'll need a few iterations before the wording is satisfatory. Can you please do a first review?

Closes https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/issues/1537