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/security-context.yaml

@@ -0,0 +1,21 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: security-context-demo
+spec:
+  securityContext:
+    runAsUser: 1000
+    runAsGroup: 3000
+    fsGroup: 2000
+  volumes:
+  - name: sec-ctx-vol
+    emptyDir: {}
+  containers:
+  - name: sec-ctx-demo
+    image: busybox:1.28
+    command: [ "sh", "-c", "sleep 1h" ]
+    volumeMounts:
+    - name: sec-ctx-vol
+      mountPath: /data/demo
+    securityContext:
+      allowPrivilegeEscalation: false