ALSA: doc: ReSTize OSS-Emulation document

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

Documentation/sound/designs/index.rst

@@ -7,3 +7,4 @@ Designs and Implementations
    channel-mapping-api
    procfile
    powersave
+   oss-emulation