doc: usb: Use new USB API for sample code
doc: usb: Use new USB API for sample code

Replace sample code, making it use newest USB API, reading code
directly from sources, and changing to the simplest sample - loopback.
CDC ACM is quite big and modifying sample would render line change
quite often.

Signed-off-by: Andrei Emeltchenko <a6565233ddc88e4fb9c66c1d70743223493f2ed4@intel.com>

ldts/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,punitvara/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,punitvara/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr