drm/xe/uapi: More uAPI documentation additions and cosmetic updates

No functional change in this patch.

Let's ensure all of our structs are documented and with a certain
standard. Also, let's have an overview and list of IOCTLs as the
very beginning of the generated HTML doc.

v2: Nits (Lucas De Marchi)

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>