Nomad `host_network` documentation and example improvments

[nickethier]

trafficstars

The Multi-Host Interface feature released in 0.12 can be tricky to configure properly as there are numerous different ways users can configure their host's network interfaces. It's also not clear from existing documentation how a host_network interfaces with some driver's networking features (ex. #8488).

The following documentation improvements are underway to help this.

• Updated examples in the docker driver documentation
• Multi-Host Interface learn guide
• New internals documentation on how to debug/understand how packets flow through Nomad's bridge networking mode, including the use of host_network.

Also cover #6873