Document CI runners troubleshoting guide

This issue is to track all steps needed to document CI runners troubleshooting with a possible resolution for each of know problems to aid with an easier recovery of infrastructure.

assigned to @ayufan

added ci documentation runbook labels

mentioned in issue #1238 (closed)

First part of documentation for shared runners: https://gitlab.com/gitlab-com/runbooks/merge_requests/214.

I think @tmaczukin wrote this in other issues so I'm closing this one.

Feel free to reopen if we need more.

closed