Update Makefile with additions from the Blender manual
The Blender manual also uses Sphinx and ReadTheDocs, and they have a nifty makefile and a number of helper scripts to keep local dev simpler. The license is also CC BY-SA 4.0 so maybe we could borrow and modify some parts to our own needs.
(Better yet if we can update make.bat along with the Makefile for Windows people.)