ASoC: doc: ReSTize overview.txt

A simple conversion from a plain text file.
Created a new subdirectory, Documentation/sound/soc, for this and
other ASoC documents.

Since the index page contains the TOC, so "Documentation" section got
removed from overview.

Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/Documentation/sound/index.rst b/Documentation/sound/index.rst
index 1f5d166..47b89f0 100644
--- a/Documentation/sound/index.rst
+++ b/Documentation/sound/index.rst
@@ -7,6 +7,7 @@
 
    kernel-api/index
    designs/index
+   soc/index
    alsa-configuration
    hd-audio/index
    cards/index