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/probe/http-liveness.yaml

@@ -0,0 +1,21 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  labels:
+    test: liveness
+  name: liveness-http
+spec:
+  containers:
+  - name: liveness
+    image: k8s.gcr.io/liveness
+    args:
+    - /server
+    livenessProbe:
+      httpGet:
+        path: /healthz
+        port: 8080
+        httpHeaders:
+        - name: Custom-Header
+          value: Awesome
+      initialDelaySeconds: 3
+      periodSeconds: 3