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.
added a note about for best results, assign everyone before the course starts.
I want to make the hybrid strategy appear just that way: as a conscious strategy. I added a mention of the default group, but am trying to make that seem like a last-resort failsafe rather than as an option that results in an identically positive student experience...