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.
We should describe the consequences of moving a student from one cohort to another. Each thread is assigned to its author's cohort at the time of its creation. If the author is moved to a different cohort, then they will lose access to any threads that they had previously created in a cohorted topic.