[mlir] Update docs on conversion and translation to LLVM

Create a new document that explain both stages of the process in a single
place, merge and deduplicate the content from the two previous documents. Also
extend the documentation to account for the recent changes in pass structure
due to standard dialect splitting and translation being more flexible.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D109605

Author: ftynse