[LLVM][TableGen] Notebook describing how to write a Python backend

This tutorial uses the dump json option to write a backend for
SQL queries.

It is based on the work of Min-Yih Hsu:
* https://github.com/mshockwave/SQLGen
* https://www.youtube.com/watch?v=UP-LBRbvI_U

I hope that having the same concepts in 3 forms will allow people
to choose the style that fits them.

The main drawback here being that it's in Python. C++ can be used
in a notebook (https://github.com/jupyter-xeus/xeus-cling) but I
decided against it for a few reasons:

* Python is the default for Jupyter, no extra installs needed.
* Having the code in a second language may help people who
  know one or the other.
* There is no upstream example of a JSON powered backend.
  (and although we would be unlikely to accept one upstream,
  I think it's a great option for quick development before
  translating into C++)

Reviewed By: myhsu

Differential Revision: https://reviews.llvm.org/D142364