boards: serpente: Update outdated links
boards: serpente: Update outdated links

The "Serpente" documentation base URL changed.

Signed-off-by: Stephanos Ioannidis <d862a8ba07ce1656f326514118a4618b42d2d2c0@nordicsemi.no>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr