Ahodges/doc/cohorts

@explorerleslie , @nasthagiri , @andy-armstrong , @gwprice , @jimabramson , @dan-f , @cahrens, @srpearce  , @WatsonEmily , @mhoeber 
The cohorts/cohort_config and cohorts/cohorts_overview files and images are pretty solid. Questions occasionally in comments.
I'll work on the section describing moderating discussions in a cohorted universe next.
Maybe a note here that you cannot delete or change the name of a cohort group once it has been defined? Instead, you can create a new cohort group and move all students from the old group to the new one. But the old one will be forever shown in the instructor dashboard....