Cleanup and implement style guidelines. (#18980)

* Reworded paragraphs to reduce ambiguity.
* Added min-kubernetes-server-version metadata.
* Converted yaml to a downloadable resource.

content/en/examples/service/networking/nginx-policy.yaml

@@ -0,0 +1,13 @@
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: access-nginx
+spec:
+  podSelector:
+    matchLabels:
+      app: nginx
+  ingress:
+  - from:
+    - podSelector:
+        matchLabels:
+          access: "true"