Add documentation for routed control plane networks

Kayobe currently supports definition of various different networks -
public, internal, tunnel, etc. These typically map to a VLAN or flat
network, with an IP subnet. When a cloud exceeds the size of a single
VLAN/subnet, this approach no longer works.

One way to resolve this is to have multiple subnets that map to a single
logical network, and provide routing between them. This is a similar
concept to neutron's routed networks, but for the control plane.

This change provides documentation for the currently tested parts of
this feature.

Change-Id: Ic06c6d4fff0fa568eb9ed3a9c30ce21c7699d965
Story: 2008180
Task: 40938