ALSA: docs: Add toctree index entry for co-processor acceleration API

ALSA: docs: Add toctree index entry for co-processor acceleration API

Sphinx reports missing toctree entry warning for co-processor
acceleration API docs:

Documentation/sound/designs/compress-accel.rst: WARNING: document isn't included in any toctree

Add the missing entry to fix the warning.

Fixes: 04177158cf98 ("ALSA: compress_offload: introduce accel operation mode")
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Link: https://patch.msgid.link/20241028111647.17378-3-bagasdotme@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>