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/pod-with-http-healthcheck.yaml

@@ -0,0 +1,20 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: pod-with-http-healthcheck
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    # defines the health checking
+    livenessProbe:
+      # an http probe
+      httpGet:
+        path: /_status/healthz
+        port: 80
+      # length of time to wait for a pod to initialize
+      # after pod startup, before applying health checking
+      initialDelaySeconds: 30
+      timeoutSeconds: 1
+    ports:
+    - containerPort: 80