docs: add migration guide for 1 -> 2.0.0.
docs: add migration guide for 1 -> 2.0.0.

dgonyeo/ignition,higebu/ignition,AlexNPavel/ignition,dgonyeo/ignition,AlexNPavel/ignition,higebu/ignition,ajeddeloh/ignition,coreos/ignition,coreos/ignition,AlexNPavel/ignition,coreos/ignition,higebu/ignition,dgonyeo/ignition,ajeddeloh/ignition,ajeddeloh/ignition