Links to setuptools documentation need to be updated

[edlanglois]

This packaging page (and possibly other documentation pages, I haven't checked) links to this setuptools guide page, which has largely been emptied of content.

It looks like the new documentation is here and links should reference this page or the appropriate userguide sub-pages.

[webknjaz]

FWIW it's best to use intersphinx references where possible. Here's a list of what's available: https://webknjaz.github.io/intersphinx-untangled/setuptools.rtfd.io/

[bhrutledge]

This feels related to #887.