Add kubelet serving certificate readiness example

[arnab-logs]

This PR adds a new example demonstrating how to use NRC to prevent workloads from scheduling on a node until kubelet has obtained its TLS serving certificate.

Includes:

• examples/kubelet-cert-readiness/: kind cluster config and manifests (RBAC, NPD ConfigMaps, NPD DaemonSet, NodeReadinessRule)
• docs/book/src/examples/kubelet-cert-readiness.md: mdBook documentation page
• docs/book/src/SUMMARY.md: adds the new page to the site navigation

Use Case

This is in reference to a slack discussion raised in the NRC community. When a node is marked Ready, kubectl exec and kubectl logs may still fail because kubelet has not yet received its TLS serving certificate, the certificate is issued separately via a CSR and its approval can be delayed on some providers.

This causes visible failures in CI/CD environments: GitLab Runner marks jobs as failed when it cannot exec into pods on a new node, and Fluentbit fails to collect logs from kubelet before the certificate is in place.

How to test

Create kind cluster

kind create cluster --config examples/kubelet-cert-readiness/kind-config.yaml

Install the CRDs and Controller

See the Installation Guide for details

Deploy the Example

kubectl apply -f examples/kubelet-cert-readiness/manifests/

Check the startup taint is applied

kubectl get nodes -o custom-columns=NAME:.metadata.name,TAINTS:.spec.taints

Check the node condition

   kubectl get node <node-name> \
    -o jsonpath='{.status.conditions[?(@.type=="KubeletServingCertNotReady")]}' | jq .

Approve the worker's pending CSR to simulate the certificate being issued

    kubectl get csr
    # find the entry with SIGNERNAME kubernetes.io/kubelet-serving and REQUESTOR system:node:<worker-name>
    kubectl certificate approve <worker-csr-name>

Check the condition again

    kubectl get node <node-name> \
    -o jsonpath='{.status.conditions[?(@.type=="KubeletServingCertNotReady")]}' | jq .

Check taint removal

   kubectl get node <node-name> -o jsonpath='{.spec.taints}'

Confirm kubectl exec works

   kubectl run test-pod --image=busybox --restart=Never -- sleep 3600
   
   kubectl exec test-pod -- echo "exec works"

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[kubernetes-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kubernetes-prow]

@arnab-logs: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-node-readiness-controller-verify-all	f87afe2	link	true	/test pull-node-readiness-controller-verify-all

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[arnab-logs]

Looks like the prow job failed on the docs link check. I verified the files are present in this PR and the failure seems to be because the links point to main where these new files don't exist yet