Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/examples/service/networking/nginx-secure-app.yaml

@@ -0,0 +1,51 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: my-nginx
+  labels:
+    run: my-nginx
+spec:
+  type: NodePort
+  ports:
+  - port: 8080
+    targetPort: 80
+    protocol: TCP
+    name: http
+  - port: 443
+    protocol: TCP
+    name: https
+  selector:
+    run: my-nginx
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: my-nginx
+spec:
+  selector:
+    matchLabels:
+      run: my-nginx
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        run: my-nginx
+    spec:
+      volumes:
+      - name: secret-volume
+        secret:
+          secretName: nginxsecret
+      - name: configmap-volume
+        configMap:
+          name: nginxconfigmap
+      containers:
+      - name: nginxhttps
+        image: bprashanth/nginxhttps:1.0
+        ports:
+        - containerPort: 443
+        - containerPort: 80
+        volumeMounts:
+        - mountPath: /etc/nginx/ssl
+          name: secret-volume
+        - mountPath: /etc/nginx/conf.d
+          name: configmap-volume