ALSA: doc: Add codecs/index.rst to top-level index

ALSA: doc: Add codecs/index.rst to top-level index

commit 088fb4ee17fc ("ALSA: doc: cs35l56: Add information about
Cirrus Logic CS35L54/56/57") added a new subdirectory with an index
file, so this file must be added to the top-level index.

Fixes: 088fb4ee17fc ("ALSA: doc: cs35l56: Add information about Cirrus Logic CS35L54/56/57")
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20250109105657.96860-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>