ALSA: doc: ReSTize timestamping document

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

Documentation/sound/designs/index.rst

@@ -7,6 +7,7 @@ Designs and Implementations
    control-names
    channel-mapping-api
    compress-offload
+   timestamping
    procfile
    powersave
    oss-emulation