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/application/guestbook/frontend-deployment.yaml

@@ -0,0 +1,29 @@
+# SOURCE: https://cloud.google.com/kubernetes-engine/docs/tutorials/guestbook
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: frontend
+spec:
+  replicas: 3
+  selector:
+    matchLabels:
+        app: guestbook
+        tier: frontend
+  template:
+    metadata:
+      labels:
+        app: guestbook
+        tier: frontend
+    spec:
+      containers:
+      - name: php-redis
+        image: gcr.io/google_samples/gb-frontend:v5
+        env:
+        - name: GET_HOSTS_FROM
+          value: "dns"
+        resources:
+          requests:
+            cpu: 100m
+            memory: 100Mi
+        ports:
+        - containerPort: 80