ALSA: doc: ReSTize HD-Audio document

The original HD-Audio.txt was already in asciidoc format, so it's a
simple conversion in the end.

A new subdirectory, Documentation/sound/hd-audio, is created and the
document is moved there with another file name to match better with
the recent Documentation tree structure.

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