Use dnsutils image for DNS debugging (#18001)

DNS debugging documentation is using a busybox image as example.
This image has different known issues with the way it resolves
the DNS names in k8s, creating confusion with users that try to
follow the docs.

The e2e tests use a custom small image ~4.3MB for DNS testing
with all the necessary tools. Also, the fact that this image is
being used for the k8s e2e testing guarantees it's compatibility.

content/en/examples/admin/dns/dnsutils.yaml

@@ -0,0 +1,14 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: dnsutils
+  namespace: default
+spec:
+  containers:
+  - name: dnsutils
+    image: gcr.io/kubernetes-e2e-test-images/dnsutils:1.3
+    command:
+      - sleep
+      - "3600"
+    imagePullPolicy: IfNotPresent
+  restartPolicy: Always