The current documentation is super old, crusty, and at times wrong. This commit
rewrites the documentation to focus on the TableGen declarative definition,
expounds on various components, and moves the doc out of Tutorials/ and into
a new top level `AttributesAndTypes.md` doc. As part of this, the AttrDef/TypeDef
documentation in OpDefinitions.md is removed.
Differential Revision: https://reviews.llvm.org/D120011
