Sylvia/docs/sfd discussions

@mhoeber @lamagnifica @catong This is finally ready for review. I accidentally added some extra images, so please ignore the ones that don't apply (I'll take them out before I merge this). I tried to create a built version but RTD inexplicably created the course staff release notes page (grr); will try again tomorrow.

Update: RTD is working! See http://draft-sfd-discussions.readthedocs.org/en/latest/.
I'd consider striking or rewording "of the course team" --- anyone can answer questions (students can answer, for example, then staff can indicate that a student's response is correct, for example -- off loads work, validates student effort, etc.)