Add AI grading architecture documentation.

Second pass at planning the AI grading architecture, incorporating feedback from conversations with @stephensanchez and @feanil.

@srpearce Is there a better way to set this up so that the table of contents makes more sense?  Right now all the top-level titles are appearing in the TOC; I'd like for all the top-level headings in `ai_grading.rst` to appear in their own section, but my understanding of sphinx is pretty weak :\

@ormsbee If you have a few minutes, I'd love to get your thoughts on this.