Add docs for how to run tests in development

[joerick]

Ref #1687

Adding additional information about running tests. I've chosen to spell this out using a venv and pytest because I've found that I often want to use extra pytest options, like -k and --capture, not to mention the CIBW_PLATFORM variable, so it feels like it's not all that useful to make out that it's as easy as nox -s tests.

[joerick]

I'll take another look at this after #1727 is resolved.

[joerick]

Took another go at this. The diff's a bit of a mess - it's easier to read the rendered version: https://cibuildwheel--1698.org.readthedocs.build/en/1698/contributing/#development

[joerick]

@henryiii any input on this before it's merged? I've decided to go with nox as the primary way to document these, and since we don't need CIBW_PLATFORM any more, things are simpler generally. And, nox is gonna get much faster very soon :)

Once this one is merged, I'm keen to get a release out.