Link to Django docs where possible

[rixx]

This PR introduces links to the Django documentation on all model fields that are Django core fields. (Yes, this leaves out fields like contrib.postgres, but it's better than nothing, imo).

[vdboor]

While I appreciate what you're trying to do here, this is not the right approach.
Instead of linking to a HTTP URL, a proper intersphinx configuration in your project will do that all for you.

This makes sure that linking like these are in fact linked to the Django documentation:
:class:`~django.contrib.auth.models.User`