Sylvia/docs/ora 65

@mhoeber @lamagnifica @stephensanchez @wedaly @gradyward Please review. This is a draft so things may be a little rough around the edges, and I have a couple of notes to myself inline.
@JAAkana We updated the messages so that they're more clear--for example, if you change the options for a rubric, you get a message that tells you to update the student training examples. (Check out the ora2 sandbox at http://studio.ora2.m.sandbox.edx.org/course/ to see the updates.) Do you still think we need this? I can think about adding a troubleshooting section if necessary.