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 <[email protected]>:
- fixed build error due to toctree in docs/LangRef/index.rst
- fixed reference to ProgRef)

Change-Id: Ifbdfa39768b8a460aae2873103d31c7b347aff00

Author: Paul C. Anagnostopoulos