Add sphinxcontrib-django for documentation

[mfosterw]

Description

The template should include a dependency on sphinxcontrib-django and include it in the extensions in docs/conf.py.

Rationale

sphinx-autodoc is, in my opinion, almost useless on its own for django, especially for models, and at minimum requires extensive configuration to be remotely readable. Also, all the documentation within the code from features like help_text is completely ignored and has to be repeated in the docs. Sphinxcontrib-django does away with all those issues and more, enabling informative documentation with minimal extra effort.

I am not affiliated with this project in any way, I have just become dependent on it for documenting my own cookiecutter project.