[SYCL][Docs] Add design for moving to the new offloading model (#8658)

The community has moved to using a different model for performing
offloading behaviors during link. This document provides details of
moving from our current offloading model which performs all of the
device link and compilation behaviors in the driver and moves these
actions into a separate tool which performs a more 'conglomerate' link
step.

Author: mdtoguchi