Enh: align tests section better with our lessons

[lwasser]

I am working on videos on test suites and task runners for our course. And the test section of our guide is dated.

We use Nox in the guide, which I love, but our lessons and template use Hatch. So this tries to clean things up, cross-link, and add the nox content to a new page, replacing the old page with Hatch (our template default).

It also makes sure we are following brand guidelines and crosslinks throughout

A few notes:

1. I used AI (CLAUDE) to help me apply brand guidelines throughout. This included setting line lengths to 79 characters for readability, which makes the pr look larger than it is.
2. I also used it to add crosslinks and header RST metadata.
3. I finally used it to review all of the text and suggest where the content was confusing.

I still need to go through and edit this again via review before it is ready.
Claude made some breaking changes to things like how we call hatch environments and define them in the pyproject.toml file. So I want to review again to catch anything else that may have broken. Because Claude was involved, I broke out commits into sets of files as I worked on them.