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/

how about:
* Each unique user who engages in one of the categories of activity increases
  the count for that category by 1. A student who completes 10 problems increases
  the count of students who tried a problem by 1 and increases the overall
  count of active students by 1.