drm/xe/doc: Include documentation about xe_assert()

Our xe_assert() macros are well documented.
Include that in master documentation.

Reviewed-by: Lucas De Marchi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Michal Wajdeczko <[e-mail]>
Signed-off-by: Rodrigo Vivi <[e-mail]>

Documentation/gpu/xe/index.rst

···
    xe_rtp
    xe_firmware
    xe_tile
+   xe_debugging

Documentation/gpu/xe/xe_debugging.rst

···
+.. SPDX-License-Identifier: (GPL-2.0+ OR MIT)
+
+=========
+Debugging
+=========
+
+.. kernel-doc:: drivers/gpu/drm/xe/xe_assert.h