[Doorlock][CATS] Missing command to give permission to flash the Arduino board

(Doc revision 48056d5a)

Before the flash_usb command, instructions to give permission to flash the board are missing.

An example of such commands is in:
https://docs.oniroproject.org/en/latest/oniro/hardware-support/boards/arduino-nano-33-ble.html#flashing-an-application

RTD links:
https://docs.oniroproject.org/projects/blueprints/doorlock.html#flash-the-software-when-built-with-bitbake
https://docs.oniroproject.org/projects/blueprints/cats.html#prepare-openthread-radio-co-processor-rcp