drm/i915: Add missing CCS documentation

Let's introduce the basic documentation about CCS.
While doing that, also removed the legacy execution flag name. That flag
simply doesn't exist for CCS and it is not needed on current context
submission. Those flag names are only needed on legacy context,
while on new ones we only need to pass the engine ID.

It is worth mention that this documentation should probably live with
the engine definitions rather than in the i915.rst file directly and
that more updates are likely need in this section. But this should
come later.

v2: Overall improvements from Matt and Tvrtko.

Fixes: 944823c9 ("drm/i915/xehp: Define compute class and engine")
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Sushma Venkatesh Reddy <sushma.venkatesh.reddy@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230926165107.23440-1-rodrigo.vivi@intel.com