[LLVM][TableGen][Jupyter] Add first tutorial notebook

This first notebook covers the basics.
* classes
* defs
* basic types
* let in various forms

Everything up to multiclass, which will be the
start of the 2nd part of the tutorial.

I'd like to keep them in logical sections as
far as possible, so they are easy to digest.

Differential Revision: https://reviews.llvm.org/D137085