minor tweaks to get sphinx build to not issue warnings

I just tried building the new sphinx docs - they look nice, but I noticed it was issuing 3 warnings.  Two were about documents that aren't in the TOC, and the third was about a missing ``_static`` directory.  This PR fixes those problems - the former by adding the ``:orphan:`` tag to the files (which tells sphinx that they aren't *supposed* to be in the TOC), and the latter by adding a tweak in the Makefile that creates an empty ``_static`` directory if it is missing.