[doc] Add package documentation for core
[doc] Add package documentation for core

TheThingsNetwork/ttn,LoRaWanSoFa/ttn,mgranberry/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,jvanmalder/ttn,LoRaWanSoFa/ttn,mgranberry/ttn,mgranberry/ttn,jvanmalder/ttn,TheThingsNetwork/ttn