ALSA: doc: ReSTize ALSA-Configuration document

A simple conversion from the text file.

Since this is the only document specific to the configurations, it's
put to the root sound subdirectory.

A section describing the obsoleted configure stuff of old alsa-driver
tarball got removed.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/index.rst

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