transparent-gateway: Document new way to build and flash RCP

Rewrite RCP build and flash section to use bitbake

We have a much simpler and quicker way to build and flash the OpenThread Radio Co-Processor now by using bitbake.

Replacing the Build and Flash the Radio Co-Processor Firmware section of the docs with the new way to flash RCP.

Replace Makerdiary nrf52840 with Arduino Nano 33 BLE as default board for RCP. Mentioning the nrf52840 as alternative board.

Remove python3-serial from requirements for flashing the zephyr mesh node

Add a note linking the troubleshooting section in case the onboarding process is stuck

Signed-off-by: Francesco Pham francesco.pham@huawei.com