[Doc]: Move Automated Tests section to workflow docs #26998

Documentation Link

https://matplotlib.org/devdocs/devel/coding_guide.html#automated-tests

Problem

This section on the automated tests is very useful and also buried in detailed guidelines and it's a mix of "writing PR" tasks and "prepping PR for merge" tasks and I think automated tests is sort of neither as it's something you may have to run a ton of times on a PR.

The only reason I found this section was because I knew it existed and had a guess at which of the three pages it might be in.

Suggested improvement

I'd find it more intuitive to put with the workflow information because that's where the other github related stuff lives:

and maybe leave an anchor link in the guidelines doc (which granted, I think needs a major overhaul #26095).