Add troubleshooting doc

There are some notes in run_qa_against_gdk.md that can help with troubleshooting but I think it would be useful to have a dedicated document.

It could include relatively basic info like "make sure you pull the latest gitlab/gitlab-{ce|ee}:nightly image before you run tests", which I sometimes forget and then wonder why I run into bugs that were fixed days ago.

It could also include instructions for how to set up and run a custom pipeline, which I just discovered can be really handy when testing CI setup and reproducing bugs that seem to only occur in CI.