Add a section to describe kubelet configuration with CAPBK

Signed-off-by: Kotaro Inoue <[email protected]>

Author: musaprg

docs/book/src/SUMMARY.md

@@ -11,6 +11,7 @@
         - [Auto Rotate Certificates in KCP](./tasks/certs/auto-rotate-certificates-in-kcp.md)
     - [Bootstrap](./tasks/bootstrap/index.md)
         - [Kubeadm based bootstrap](./tasks/bootstrap/kubeadm-bootstrap/index.md)
+            - [Kubelet configuration](./tasks/bootstrap/kubeadm-bootstrap/kubelet-config.md)
         - [MicroK8s based bootstrap](./tasks/bootstrap/microk8s-bootstrap.md)
     - [Upgrading management and workload clusters](./tasks/upgrading-clusters.md)
     - [External etcd](./tasks/external-etcd.md)