Improve documentation on Linkerd 2.x for Kubernetes NetworkPolicy

@kflynn is already working on this.

Since Sep 25, 2025.

### What problem are you trying to solve?

In hardening a Kubernetes cluster, using a NetworkPolicy to restrict ingress between namespaces is useful.

I only found documentation here: https://ihcsim.medium.com/linkerd-2-x-with-network-policy-2657103333ca

Which pointed me in the direction to get this functionality working.

Specifically (though this could be tightened to port 443 as mentioned in the webpage above).

```
kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
name: allow-meshed-ingress
namespace: linkerd
spec:
ingress:
- from:
- ipBlock:
cidr: X.X.X.X/20
podSelector: {}
policyTypes:
- Ingress
---
kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
name: allow-meshed-ingress
namespace: linkerd-viz
spec:
ingress:
- from:
- ipBlock:
cidr: X.X.X.X/20
podSelector: {}
policyTypes:
- Ingress
```

And, in the case of the namespace we want permission for linkerd to to access:

```
apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
name: allow-linkerd-ingress
namespace: emojivoto
spec:
ingress:
- from:
- namespaceSelector:
matchLabels:
linkerd.io/control-plane-ns: linkerd
podSelector:
matchLabels:
linkerd.io/control-plane-ns: linkerd
podSelector: {}
policyTypes:
- Ingress
```

Since a default restricted namespace allows no ingress between namespaces.

### How should the problem be solved?

Some documentation to help improve security with linkerd. It is of course, optional if you want to use a NetworkPolicy to restrict traffic.

### Any alternatives you've considered?

No other solution known

### How would users interact with this feature?

Maybe improve the `emojivoto` demo app, to add more functionality such as NetworkPolicy, etc.

### Would you like to work on this feature?

maybe