drm/i915: Add missing GSCCS documentation

Introduce the basic documentation about GSC CS.

This "GPU Basics" section is focused on explaining the hardware
rather than the driver/uapi, so let's make sure GSC is also
properly documented here.

v2: Fixes from Matt: typos and acronym.

Fixes: 5fd974d1 ("drm/i915/mtl: add initial definitions for GSC CS")
Suggested-by: Matt Roper <matthew.d.roper@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926175554.25968-1-rodrigo.vivi@intel.com