Document Pod and Service network configuration

[NicolasT]

Cfr. https://github.com/scality/metalk8s/issues/2137#issuecomment-572831909 (and to some extent https://github.com/scality/metalk8s/issues/415 which was about MetalK8s 1.x):

it appears we currently don't document the networks.pods and networks.services settings of BootstrapConfiguration, which allows overriding the defaults (10.233.0.0/16 and 10.96.0.0/12, respectively) if these collide with existing infrastructure.

This should be

• Documented
• We should configure one of the CI deployments to use non-default networks to ensure we never accidentally rely on these values to be set to their defaults

[Ebaneck]

The documentation part of this issue was handled in https://github.com/scality/metalk8s/issues/2114