Add Jupyter notebooks using nbsphinx

[The9Cat]

• The first two Introduction notebooks are imported into the ReadTheDocs pyEXP examples chapter, rather than the markdown excerpts.
• The nbsphinx extension is now a dependency and added to both conf.py and requirements.txt
• The Doxygen C++ documentation is updated

[The9Cat]

Okay, issues seem to be sorted out now.

I'm not 100% happy with the intro wording to the pyEXP examples although it's probably good enough. Feel free to edit.

[The9Cat]

Got the copy buttons to work in the inline Jupyter code blocks. So we have 100% satisfied the referee's requests at this point.

[michael-petersen]

Amazing, the pages look so good! Love that we can see input and output.

This addresses openjournals/joss-reviews#7302 (comment).

[The9Cat]

It isn't 100% 'plug and play'. For example, one would need the data in the repo to really do the examples, even with the copy buttons. I suppose, in retrospect, that's why I whittled down the snippets in ReadTheDocs to standalone bits. Still, I suppose seeing a full up notebook with output is better, even if it would require the pyEXP-examples repo to run.