Integration test docs
Created by: felixfbecker
Closes #12178.
Restructures, overhauls, and extends our testing docs for integration tests. Some things for e2e tests were outdated, some apply to integration tests too. I tried to create a structure that makes sense (and will be shown in the automatic ToC on the right on docs.sourcegraph.com).
I also made some small DX improvements I noticed while writing/reading the existing the docs.