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
Consider dropping the explicit version number since you are directing the user to the devstack doc which already specifies the version and updates the version periodically.  We don't want to put something in the doc that by its volatility will make the doc out-of-date.