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.
@lamagnifica we allow you to manually assign students to any cohort, even if it is an auto-cohort group. An example use case is if a student is assigned to a manual cohort group accidentally, and you want to remove him from that group and add the student to an auto-cohort group. Or if one of the auto cohort groups has too few students in it, and you need to move all students out of one auto cohort group into another.