drm/xe: Introduce GGTT documentation

drm/xe: Introduce GGTT documentation

Document xe_ggtt and ensure it is part of the built kernel docs.

v2: - Accepted all Michal's suggestions
    - Rebased on top of new set_pte per platform/wa function pointer
v3: - Typos and other acronym fixes (Michal)

Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com> #v1
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240821193842.352557-2-rodrigo.vivi@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>