Making the new TableGen documentation official and marking the old file as
"Moved". Also, reverting the original LangRef as the normative formal
description of the language, while keeping the "new" LangRef as LangIntro
for the less inlcined to reading language grammars.

We should remove TableGenFundamentals.rst one day, but for now, just a
warning that it moved will have to do, while we make sure there are no more
links to it from elsewhere.

llvm-svn: 205289

Fun fact: looking at the TableGen code (around TGParser.cpp:1166), the
only difference in handling is that adjacent regular string literals are
concatenated in the parser.

llvm-svn: 201035

Code fragments are just fancy string literals.

llvm-svn: 201034

llvm-svn: 197757

Boilerplate is often the hardest part of getting started with these
kinds of things, so throw them a bone.

llvm-svn: 173969

Before we learned about :doc:, we used :ref: and put a dummy link at the
top of each page. Don't do that anymore.

This fixes PR14891 as a special case.

llvm-svn: 172162

llvm-svn: 171508

llvm-svn: 168380

llvm-svn: 158904

llvm-svn: 158903