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>

Documentation/sound/hd-audio/index.rst

+HD-Audio
+========
+
+.. toctree::
+   :maxdepth: 2
+
+   notes

Documentation/sound/index.rst

@@ -6,6 +6,7 @@ Linux Sound Subsystem Documentation
    :maxdepth: 2
 
    kernel-api/index
+   hd-audio/index
 
 .. only::  subproject