[NFC][doc] Rewrite introduction to TableGen to be more modern and approachable

[johnwbyrd]

I've just helped develop a novel LLVM backend (LLVM-MOS), and these changes include what I wish someone had told me about TableGen before I got started. I've purposely written in an approachable style, which is sorely lacking in the current state of the TableGen docs.

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[johnwbyrd]

@Paul-C-Anagnostopoulos