Re-factor TableGen docs

Re-factor TableGen docs

This is mainly a movement of content around to give place to new content
allowing different people to add bits to it in the right place. There is some
new content, but mostly to fill the gaps left by text movement.

I'm dropping the old syntax documentation as it has the problem of being
quickly outdated by changes and largely unnecessary to people not involved
in creating the language, but using it, which is the whole point of the
documentation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204351 91177308-0d34-0410-b5e6-96231b3b80d8