docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/Kconfig

@@ -74,7 +74,7 @@ config USB
 	  After choosing your HCD, then select drivers for the USB peripherals
 	  you'll be using.  You may want to check out the information provided
 	  in <file:Documentation/usb/> and especially the links given in
-	  <file:Documentation/usb/usb-help.txt>.
+	  <file:Documentation/usb/usb-help.rst>.
 
 	  To compile this driver as a module, choose M here: the
 	  module will be called usbcore.