ALSA: documentation: Add description for USB MIDI 2.0 gadget driver

The USB MIDI 2.0 gadget driver is now supported for 6.6 kernel, and
here we show a brief instruction how to enable and use it.

Link: https://lore.kernel.org/r/20230824075108.29958-5-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>