When using a network as an additional network in the OCP cluster
that has an IPv6 Subnet, that subnet must be included in the router
to provide router advertisements, otherwise the interface wouldn't get
an address configured. This happens because the network manager is
configured with method=auto.
The `MachinesSubnet` field has been reshaped as `controlPlanePort`,
this commit updates the docs to ensure `controlPlanePort` is used.
Also, this commit adds dual-stack documentation.