config.txt: update documentation URL
config.txt: update documentation URL

KingBrewer/pi-gen,WLANThermo/pi-gen,DigitaleGesellschaft/pi-gen,rkubes/pi-gen,barnacleos/build,timcoote/Pumpkin-gen,drothmaler/pi-gen,BenjaminEHowe/keyringpi-gen,brindosch/pi-gen,jacen92/pi-gen,BlokasLabs/modep,cfstras/pi-gen,BlokasLabs/modep,RPi-Distro/pi-gen,timcoote/Pumpkin-gen,jannik-kramer/Mean,8devices/pi-gen,timcoote/Pumpkin-gen