Add -a flag to sphinx-build.

Add -a flag to sphinx-build. If you add new sections to the ToCTable, this needs reflected on all pages of the menu, instead of having to make white-space changes to all documents, just so that ToCTable updates propagate.

To see the problem, have a set of documents nested a 2 or more levels deep. Build on RtD.  Then add another root document to the set.  You'll notice that while browsing some of the other documents in the set, your newly added document isn't always visible when it should be.  

This commit should resolve this, **if** the "build" button on RtD executes its commands from this Makefile.