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'm thinking of breaking this up into separate topics (one for adding, one for editing/deleting). There's not a whole lot of info for editing/deleting, but right now readers have to scroll way down to the bottom of the page to find it. Thoughts? Or I could also just add a quick intro here with links to the different sections later in the topic...