Add make watch to documentation

I use this in other Python projects to make writing the documentation faster & easier.

Basically, after installing all dependencies in requirements.txt, instead of having to build and then open or serve the docs from the build directory, after this MR the make watch command can be used to:

1. start a web server for the documentation at http://localhost:8000
2. re-build the HTML if anything in the documentation sources change (i.e. if you edit any .rst file, for instance)

This way you can just keep a tab open in the served page, edit in another monitor, and it should automagically refresh for you (or you press F5).

-Bruno