Missing explanation of the NetworkAttachmentDefinition

[Akrog]

There is no explanation of the NetworkAttachmentDefinition on the Backend Services Deployment document but then on the OVN data migration we talk about it as if it's something that we should know about.

These are automatically done following the Development Environment doc but it may be good to mention it explicitly so we know that it is done when we go through the adoption steps.

Note: I was having trouble because I couldn't find any networkattachmentdefinition resources, turns out I had to use the whole networkattachmentdefinition.k8s.cni.cncf.io instead.