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/inject/dapi-envars-pod.yaml

@@ -0,0 +1,38 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: dapi-envars-fieldref
+spec:
+  containers:
+    - name: test-container
+      image: k8s.gcr.io/busybox
+      command: [ "sh", "-c"]
+      args:
+      - while true; do
+          echo -en '\n';
+          printenv MY_NODE_NAME MY_POD_NAME MY_POD_NAMESPACE;
+          printenv MY_POD_IP MY_POD_SERVICE_ACCOUNT;
+          sleep 10;
+        done;
+      env:
+        - name: MY_NODE_NAME
+          valueFrom:
+            fieldRef:
+              fieldPath: spec.nodeName
+        - name: MY_POD_NAME
+          valueFrom:
+            fieldRef:
+              fieldPath: metadata.name
+        - name: MY_POD_NAMESPACE
+          valueFrom:
+            fieldRef:
+              fieldPath: metadata.namespace
+        - name: MY_POD_IP
+          valueFrom:
+            fieldRef:
+              fieldPath: status.podIP
+        - name: MY_POD_SERVICE_ACCOUNT
+          valueFrom:
+            fieldRef:
+              fieldPath: spec.serviceAccountName
+  restartPolicy: Never