ASoC: docs: fix TAS675x doc build warnings

ASoC: docs: fix TAS675x doc build warnings

Add tas675x.rst to the index file and extend the heading underline
to avoid build warnings:

Documentation/sound/codecs/tas675x.rst: WARNING: document isn't included in any toctree [toc.not_included]
Documentation/sound/codecs/tas675x.rst:659: WARNING: Title underline too short.
Overtemperature Shutdown (0x87)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ [docutils]

Fixes: ba46edca354e ("Documentation: sound: Add TAS675x codec mixer controls documentation")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patch.msgid.link/20260429225751.1995224-1-rdunlap@infradead.org
Signed-off-by: Mark Brown <broonie@kernel.org>