Carol/docs/doc 803

Create wiki documentation: topic for course authors and shared topic for students.
@mhoeber @lamagnifica @srpearce @JAAkana @explorerleslie please review. 
I felt obliged to mention this here, to provide an understanding of where users find themselves when they accidentally navigate up to that edX-wide wiki level, thinking that "Wiki" in the breadcrumbs means just their course wiki. I can try to set the context better either in this pass, or a future enhancement.