[flang][docs] Add lowering design doc for parameterized derived-type

This document aims to give insights at the representation of parameterized
derived-type (PDTs) in FIR and how PDTs are lowered to FIR and interact
with the runtime.

Reviewed By: jeanPerier, klausler

Differential Revision: https://reviews.llvm.org/D133096

Author: clementval