Reorg of cohorts and discussion topics

Pulled the reorg of cohort and discussion topics out of previous branch DOC-1077 so that it is no longer dependent on that open source PR. Topics about setting up and managing discussion now separated from the cohort configuration section.
@mhoeber @lamagnifica @srpearce @explorerleslie please give thumbs if you're OK with the reorg.
Would like to create new doc for cohorted courseware on top of this file organization.
My thinking was that the heading "Options for Assigning Students to Cohort Groups" logically leads to subheads that list in a parallel way the different options for assigning students to cohorts: Options for assigning: - automated, manual, hybrid. The focus is on which way the course team would choose -- then they see the task/advantage.