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 don't feel like the emphasis is moving away from tasks. I mostly just added orientation information - we didn't have a "what is this thing I'm looking at?" explanation anywhere, and I think that's going to be a student's first question when he's looking at the *Discussion* page for the first time. The sections other than this anatomy section all have activity titles and are focused around actions; the user hopefully won't have any problem finding information about how to accomplish particular goals.

I agree that 8 things is a lot at once, but I'm not sure how we would divide up this information. I meant this to be a one-stop place where a user can go if he has a question about any part of the UI, like the orientation diagrams that a lot of owner's manuals have.