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.
nit: Course authors would almost never name their discussion topics this way, preferring to put them in human-readable format -- "Example Topic Name" -- because whatever is there shows up in the category sidebar and in posts. Maybe format these like in the other docs for discussion topics? http://edx.readthedocs.org/projects/edx-partner-course-staff/en/latest/running_course/discussions.html?highlight=discussion%20topics#organizing-discussions