docs: mention more projects that use Nox

[henryiii]

I've corrected the link to pipx (part of pypa now), as well as added several projects that use nox. Feel free to filter if you want. Current additions:

• pypa/pip
• pypa/cibuildwheel
• pypa/packaging
• pypa/manylinux
• scikit-build (just linked to scikit-build/scikit-build, though scikit-build/cmake uses it too and (eventually) scikit-build/ninja)
• scikit-hep's developer page, which links the scikit-hep projects that use it (scikit-hep/cookie is one of note, along with others)

[cjolowicz]

Thanks!