New TableGen Programmer's Reference document

This new TableGen Programmer's Reference document replaces the current Language Introduction and Language Reference documents. It brings all the TableGen reference information into one document.

As an experiment, I numbered the sections in the document. See what you think about that.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D85838

(changes by Nicolai Hähnle <nicolai.haehnle@amd.com>:
- fixed build error due to toctree in docs/LangRef/index.rst
- fixed reference to ProgRef)

Change-Id: Ifbdfa39768b8a460aae2873103d31c7b347aff00