Add link to .pypirc specification (GH-20680) (#21019)

Add link to .pypirc specification (GH-20680) (#21019)

Related to https://github.com/pypa/twine/issues/638 and https://github.com/pypa/packaging.python.org/issues/730, I wrote a spec based on the one that was removed in https://github.com/python/cpython/pull/13087. However, a Google search for "pypirc" turned up at least one [blog post](https://truveris.github.io/articles/configuring-pypirc/) that links to https://docs.python.org/3/distutils/packageindex.htmlGH-the-pypirc-file, which now just links to this document. So, I thought a link to the spec would be handy.

Automerge-Triggered-By: @jaraco
(cherry picked from commit af157fad286c00ff204e86d8556648cbb53ba99e)

Co-authored-by: Brian Rutledge <brian@bhrutledge.com>
Co-authored-by: Brian Rutledge <brian@bhrutledge.com>