Carol/docs/doc 803

Create wiki documentation: topic for course authors and shared topic for students.
@mhoeber @lamagnifica @srpearce @JAAkana @explorerleslie please review. 
is there a difference between an "edX user" and a "public user"?
it might be useful to use phrases with registered and enrolled in them instead, like "anyone with a registered user account" vs. "a user who is enrolled in your course"