doc: man-page for the sound command

Provide a man-page for the sound command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 0bc8288..bbd40a6 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -73,6 +73,7 @@
    cmd/scp03
    cmd/setexpr
    cmd/size
+   cmd/sound
    cmd/temperature
    cmd/tftpput
    cmd/true