ALSA: doc: ReSTize Audigy-mixer.txt

Another simple conversion from a plain text file.
Put to cards subdirectory.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/cards/index.rst

@@ -7,3 +7,5 @@ Card-Specific Information
    joystick
    cmipci
    sb-live-mixer
+   audigy-mixer
+