Add see-also links for some extension module helpers #325

Extension module patterns guide: https://pythonextensionpatterns.readthedocs.io/en/latest/index.html
2/3 compatible extensions: https://py3c.readthedocs.io/en/latest/

These may also be worth adding as "See Also" links to the CPython docs (or, alternatively flesh out PyPUG's extension module discussion, and add a link to that).