docs: add ipython directive extension

Add an extension providing direct execution of Python code samples along
with the ability to embed plots.
docs: add requirements files for building

In addition to the setup.py requirements, we also need to create a
requirements.txt file for the benefit of readthedocs.
docs: add empty _static dir

The automatic plot-embedding support from the ipython directive will
fail unless there is a _static dir available. We don't currently use the
_static dir. Create it and add a .gitignore which ignores gen-*.png
files. As long as we remember to make generated figures match that
filename glob we won't have git status-spam.
docs: add ipython sample to registration