Ahodges/doc/doc 792

Initial, proof of concept draft, focusing initially on enrollment chapters. @shnayder , @clintonb , @dsjen , @mulby , @brianhw , @rocha , @JAAkana , @WatsonEmily , @stroilova please feel free to comment anytime on this PR from now through release.

An HTML draft is now available! http://draft-edx-insights.readthedocs.org/en/latest/

Further, computations of different metrics within the Insights app occur at different times, also resulting in slightly different numbers.