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-volume.yaml

@@ -0,0 +1,38 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: kubernetes-downwardapi-volume-example
+  labels:
+    zone: us-est-coast
+    cluster: test-cluster1
+    rack: rack-22
+  annotations:
+    build: two
+    builder: john-doe
+spec:
+  containers:
+    - name: client-container
+      image: k8s.gcr.io/busybox
+      command: ["sh", "-c"]
+      args:
+      - while true; do
+          if [[ -e /etc/podinfo/labels ]]; then
+            echo -en '\n\n'; cat /etc/podinfo/labels; fi;
+          if [[ -e /etc/podinfo/annotations ]]; then
+            echo -en '\n\n'; cat /etc/podinfo/annotations; fi;
+          sleep 5;
+        done;
+      volumeMounts:
+        - name: podinfo
+          mountPath: /etc/podinfo
+  volumes:
+    - name: podinfo
+      downwardAPI:
+        items:
+          - path: "labels"
+            fieldRef:
+              fieldPath: metadata.labels
+          - path: "annotations"
+            fieldRef:
+              fieldPath: metadata.annotations
+