Add Jupyter-Book documentation

[jdebacker]

This PR addresses Issue #130 and adds Jupyter-Book documentation for Tax-Brain.

[jdebacker]

Screenshot of opening chapter:

[andersonfrailey]

This looks great, @jdebacker! I'll merge whenever you say it's good to go.

[jdebacker]

@andersonfrailey This PR is ready to go. But for this to deploy to Github Pages, though, you'll need to create a gh-pages branch in this repo. I forget if you do this by going to "Settings" for this repo. If not that, I think these instructions should get you there. You might create the gh-pages branch, then merge this after that.