[acc] OpenACC dialect design philosophy and details (#75548)

This document captures the design philosophy of the acc dialect. It also
shares the rationale behind the design and implementation of various
operations - and ties that back to the dialect design goals.

Co-authored-by: Valentin Clement <[email protected]>
Co-authored-by: Slava Zakharin <[email protected]>

Author: 3 people