Build API docs using sphinx autodoc
This requires turning off "nit-picky" mode due to references to types unknown
to sphinx without getting intersphinx involved (e.g. str, django.db.models.Model,
I added a little extension to napoleon to support our "Fields" and "Relations"
sections that we use in our Django models, which made it easy to test different
Sphinx methods of exposing class attributes, and also lets us use napoleon. :)