ASoC: doc: Add documentation for SOC USB

With the introduction of the soc-usb driver, add documentation highlighting
details on how to utilize the new driver and how it interacts with
different components in USB SND and ASoC.  It provides examples on how to
implement the drivers that will need to be introduced in order to enable
USB audio offloading.

Signed-off-by: Wesley Cheng <quic_wcheng@quicinc.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409194804.3773260-20-quic_wcheng@quicinc.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>