Improve Network Policy setup & doc

[jotak]

When the OVN-Kubernetes CNI is detected, a network policy is deployed by default, restricting the access from and to the NetObserv namespaces. It's different for other CNIs, because some features are implementation-specific (e.g: how host-network traffic is handled by policies). We should at least support netpols in kind (see https://github.com/kubernetes-sigs/kind/releases/tag/v0.24.0), and do a best effort otherwise - with community contributions welcome.

Another aspect is to better warn for missing policy, and improve the documentation.

• Investigate / implement what's missing for network policy on Kind.
• Raise warnings / degrade conditions when no network policy is detected.
• Better document the required ACLs for NetObserv, for users using CNIs that don't support the embedded policy.
• Document the risks associated to not having a network policy.