Cohorted courseware: Reword cohorts and content experiment overview/config in preparation

Preparation for cohorted courseware doc: separate out the conceptual and configuration content that applies to cohorted content groups vs content experiments; update terminology for the redesigned group configurations page ("experiment groups").


It was a good task-oriented title but more appropriate when the section was mostly about dividing discussions to build course communities. I felt that it didn't apply so well to the modified section, and might be misleading, as we don't really talk much about building course communities in the following topics other than in the intro paragraph.