Update/tune Advanced deck section, content, headings

Reorganizes a few topics and groups them into more unified headings. Removes topics yet to be explored/detailed in a manner applicable for Kit.
I'm torn on deleting this, though I know some things need to go. I really liked showing off the API in classes lately as it demonstrates how powerful the platform really is, beyond mere Git.  In fact, one of my goals for the advanced curriculum is to weave more GitHub into it, such as leveraging the commit status API, setting the refspec to retrieve `*/pulls/*`, and using more shortcuts like comparing arbitrary commits via the `/compare` URL syntax.