docs: usb: Add documentation for the UVC Gadget

The UVC Gadget function has become quite complex, but documentation
for it is fairly sparse. Add some more detailed documentation to
improve the situation.
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230308165213.139315-1-dan.scally@ideasonboard.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/index.rst

@@ -16,6 +16,7 @@ USB support
     gadget_multi
     gadget_printer
     gadget_serial
+    gadget_uvc
     gadget-testing
     iuu_phoenix
     mass-storage