ASoC: doc: ReSTize jack.txt

A simple conversion from a plain text file.
Acked-by: NMark Brown <broonie@kernel.org>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

Documentation/sound/soc/index.rst

@@ -15,3 +15,4 @@ The documentation is spilt into the following sections:-
    machine
    pops-clicks
    clocking
+   jack

Documentation/sound/alsa/soc/jack.txt → Documentation/sound/soc/jack.rst

+===================
 ASoC jack detection
 ===================