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/security/hello-apparmor.yaml

@@ -0,0 +1,13 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: hello-apparmor
+  annotations:
+    # Tell Kubernetes to apply the AppArmor profile "k8s-apparmor-example-deny-write".
+    # Note that this is ignored if the Kubernetes node is not running version 1.4 or greater.
+    container.apparmor.security.beta.kubernetes.io/hello: localhost/k8s-apparmor-example-deny-write
+spec:
+  containers:
+  - name: hello
+    image: busybox:1.28
+    command: [ "sh", "-c", "echo 'Hello AppArmor!' && sleep 1h" ]