Pdpinch/documentation updates

WIP. Not ready to merge yet. 

@cgee or @bdero can you review the installation instructions, esp. part 3:

https://github.com/mitodl/edx-sga/blob/pdpinch/documentation-updates/README.rst#installation
Because it is a django app it still does need to be added to `INSTALLED_APPS` or `OPTIONAL_APPS` regardless of prod or "trying out"