Refactor testing page and add link checking documentation
Refactors testing page and adds documentation for link-checking Make targets.
I also notice we weren't routinely running yamllint
, so we may as well!
Preview of page: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/bb2833404a2e1afebefe4df50d167075d644a86f/doc/testing.md
Edited by Evan Read