ALSA: docs: Add MIDI 2.0 documentation

Add the brief document for describing the MIDI 2.0 implementation on
Linux kernel.  Both rawmidi and sequencer API extensions are
described.
Acked-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-38-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/designs/index.rst

@@ -15,3 +15,4 @@ Designs and Implementations
    oss-emulation
    seq-oss
    jack-injection
+   midi-2.0