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/policy/privileged-psp.yaml

@@ -0,0 +1,27 @@
+apiVersion: policy/v1beta1
+kind: PodSecurityPolicy
+metadata:
+  name: privileged
+  annotations:
+    seccomp.security.alpha.kubernetes.io/allowedProfileNames: '*'
+spec:
+  privileged: true
+  allowPrivilegeEscalation: true
+  allowedCapabilities:
+  - '*'
+  volumes:
+  - '*'
+  hostNetwork: true
+  hostPorts:
+  - min: 0
+    max: 65535
+  hostIPC: true
+  hostPID: true
+  runAsUser:
+    rule: 'RunAsAny'
+  seLinux:
+    rule: 'RunAsAny'
+  supplementalGroups:
+    rule: 'RunAsAny'
+  fsGroup:
+    rule: 'RunAsAny'