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.
I'd move the bullets about the way students are divided into cohorts above the bullet about discussion topics (and arrange the rest of this section in the same way; i.e., current 10.1.2 would become 10.1.1 and vice versa). That way the info about what cohorts are comes before the info about what they can be used for.