networking: comprehensive document that explains networking configuration options

[dustymabe]

We need a highlevel document that explains the options users have to configure networking. Because Networking may be required in the initramfs to fetch the ignition configuration it can't be the only source of truth for networking information, thus we have kargs, coreos-installer --copy-network, and side channels on some platforms (VMWare). It would be good if we have a highlevel document that explains the relationship between these options and when the user should choose one over the other.

We should also explain the "conditional" nature of initramfs networking since we recently implemented a feature that would only bring up networking in the initramfs if it's determined to be needed to fetch the ignition config, or if the user explicitly asked for it.

Some good material for this could come from openshift/enhancements#394

[jlebon]

We could explain this stuff in the "Configuring Network" heading itself and then have more specific examples of networking configs in subheadings.

[dustymabe]

I think I'm going to try to work on this. along with complex networking configuration.

[dustymabe]

#204