boards/nrf52840dongle docs: Introduce "quick start" section
boards/nrf52840dongle docs: Introduce "quick start" section

to mention the board name; also to point to a good first example to use
with this board.

Closes: https://github.com/RIOT-OS/RIOT/issues/15654

kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,miri64/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,authmillenon/RIOT,miri64/RIOT,kaspar030/RIOT,authmillenon/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,OlegHahm/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,kaspar030/RIOT,miri64/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,authmillenon/RIOT