drm/doc: gpusvm: Add GPU SVM documentation

Add documentation for agree upon GPU SVM design principles, current
status, and future plans.

v4:
 - Address Thomas's feedback
v5:
 - s/Current/Basline (Thomas)
v7:
 - Add license (CI)
 - Add examples for design guideline reasoning (Alistair)
 - Add snippet about possible livelock with concurrent GPU and and CPU
   access (Alistair)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Alistair Popple <apopple@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306012657.3505757-33-matthew.brost@intel.com