Petr's comment in parent epic:
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The backend of the feature is tracked in https://issues.redhat.com/browse/CNV-17002.

To cover this with docs, we should introduce a page similar to the existing bridge docs: https://docs.openshift.com/container-platform/4.13/virt/virtual_machines/vm_networking/virt-attaching-vm-multiple-networks.html#virt-creating-linux-bridge-network-attachment-definition

The UI procedure for OVN should be similar to the one of bridge, except we would select a different "Network Type" and there would be no equivalent of steps 5-7.
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I think you could reuse "modules/virt-creating-bridge-nad-web.adoc" with ifdefs for OVN. It can be placed in the new OVN assembly (CNV-18089).