(📚) document pygments

when pygments is installed, pytest will color diffs in the output. this should be documented, or provided as an extra

i spent a while trying to figure out why pytest was acting different in different projects

See #7683. As mentioned there, I'd say the right route is to make it a normal dependency. Given that it has been an "experimental" feature for almost 5 years at this point, I don't think there are any plans moving away from it.