Updated docs makefile

This PR updates docs/conf.py and docs/Makefile such that the Makefile is just a shell around python -m sphinx. This should simplify generation of documentation on external systems, such as readthedocs.io.

This required moving the conversion of the examples from python into RST into the conf.py.

I also (re)instated support for the build_sphinx setup command, so that documentation can be built directly from the setup.py.