Provide mechanism or guidance on how to integrate networks

### Proposal

Nomad allows operators to create multiple custom CNI networks and have allocations pick and choose which ones to use. But mixing multiple networks may not work out-of-the-box without additional manual configuration from users and it may not be clear what is needed.

### Use-cases

Create multiple CNI networks and have them communicate with each other.

### Attempted Solutions

N/A

> **Note**
> Issue extracted from https://github.com/hashicorp/nomad/issues/13824. Refer to that issue for more context.

Start by reading the referenced issue 13824 for the missing context around integrating multiple custom CNI networks. Determine whether the outcome should be a Nomad mechanism or user guidance, with completion defined as enabling or clearly documenting communication between allocations attached to different networks.

Written by the indexing model from the issue text.