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/pods/storage/pv-pod.yaml

@@ -0,0 +1,20 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: task-pv-pod
+spec:
+  volumes:
+    - name: task-pv-storage
+      persistentVolumeClaim:
+        claimName: task-pv-claim
+  containers:
+    - name: task-pv-container
+      image: nginx
+      ports:
+        - containerPort: 80
+          name: "http-server"
+      volumeMounts:
+        - mountPath: "/usr/share/nginx/html"
+          name: task-pv-storage
+
+