docs: NodePort XDP on Azure

[gandro]

This adds a basic section with details on how to run NodePort XDP on
Azure. There are two things needed to get native XDP on a Azure VM:

• The VM needs to be have "Accelerated Networking", which attaches
  an native XDP capable mlx4/mlx5 NIC.
• Because the above VF card acts as a slave to the Hyper-V NIC on
  Azure, the running kernel must in addition also have native XDP
  for the hv_netvsc driver. This is available in Linux >= 5.6 [1].

Unfortunately, this means that running NodePort XDP on e.g. AKS is not
straightforward, as none of the provided VM images right now offer
native XDP for hv_netsvc. There is an open issue to add backport this
to Ubuntu, which is likely to land in the next few weeks [2].

Beacuse we cannot use the stock images for now, I have therefore
validated the setup on a self-managed Kubernetes cluster set up
with kubeadm on Flatcar Container Linux Edge. This one of few
Azure supporting distributions with kernel 5.6+. It runs NodePort XDP
and passes the connectivity-check.yaml with the helm install
command provided in the section. I used Azure IPAM, as I was not able
to convince the underlying vnet forward the pod-to-pod traffic
otherwise.

Because my setup is rather ad-hoc and therefore not recommended for
production use, we abstain from a step-by-step guide until native XDP
for hv_netsvc is available in official Azure images.

[1] https://lore.kernel.org/netdev/1579816355-6933-2-git-send-email-haiyangz@microsoft.com/
[2] https://bugs.launchpad.net/ubuntu/+source/linux-azure/+bug/1877654

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