ALSA: doc: ReSTize Joystick document

A conversion from a simple text file.
A new subdirectory, cards, was created to contain the card-specific
information like this one.

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

Documentation/sound/index.rst

@@ -9,6 +9,7 @@ Linux Sound Subsystem Documentation
    designs/index
    alsa-configuration
    hd-audio/index
+   cards/index
 
 .. only::  subproject