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.
This is wrong! You don't put the discussion topic names, you put their id's (as defined in Discussion topic mapping). Please change these to something that clearly look like ID's and NOT names, as this was one of the areas of confusion for the manual testers.