Document the conversion into the LLVM IR dialect

Add a documentation page on the key points of the conversion to LLVM IR.  This
focuses on the aspects of conversion that are relevant for integration of the
LLVM IR dialect (and produced LLVM IR that is mostly a one-to-one translation)
into other projects.  In particular, it describes the type conversion rules and
the memref model supporting dynamic sizes.

PiperOrigin-RevId: 235190772