apiextensions release v0.3.1

Added

• Modified docs for the G8sControlPlane CRD
• Add property descriptions to AWSMachineDeployment in infrastructure.giantswarm.io/v1alpha2.
• Add property descriptions to AWSControlPlane in infrastructure.giantswarm.io/v1alpha2.

Changed

• Change release CR back to be cluster scoped.
• Make more CR status fields omitempty.
• Make CR status fields optional.

Fixed

• Fix mistake in the main description of G8sControlPlane in infrastructure.giantswarm.io.