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/mongodb/mongo-deployment.yaml

@@ -0,0 +1,31 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: mongo
+  labels:
+    app.kubernetes.io/name: mongo
+    app.kubernetes.io/component: backend
+spec:
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: mongo
+      app.kubernetes.io/component: backend
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app.kubernetes.io/name: mongo
+        app.kubernetes.io/component: backend
+    spec:
+      containers:
+      - name: mongo
+        image: mongo:4.2
+        args:
+          - --bind_ip
+          - 0.0.0.0
+        resources:
+          requests:
+            cpu: 100m
+            memory: 100Mi
+        ports:
+        - containerPort: 27017

content/zh-cn/examples/application/mongodb/mongo-service.yaml

@@ -0,0 +1,14 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: mongo
+  labels:
+    app.kubernetes.io/name: mongo
+    app.kubernetes.io/component: backend
+spec:
+  ports:
+  - port: 27017
+    targetPort: 27017
+  selector:
+    app.kubernetes.io/name: mongo
+    app.kubernetes.io/component: backend