[mlir] Move LLVM Dialect Op documentation to ODS

This was long overdue. The initial documentation for the LLVM dialect was
introduced before ODS had support for long descriptions. This is now possible,
so the documentation is moved to ODS, which can serve as a single source of
truth. The high-level description of the dialect structure is updated to
reflect that.

Depends On: D93315

Reviewed By: rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D93425