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/access/endpoints-aggregated.yaml

@@ -0,0 +1,18 @@
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  annotations:
+    kubernetes.io/description: |-
+      Add endpoints write permissions to the edit and admin roles. This was
+      removed by default in 1.22 because of CVE-2021-25740. See
+      https://issue.k8s.io/103675. This can allow writers to direct LoadBalancer
+      or Ingress implementations to expose backend IPs that would not otherwise
+      be accessible, and can circumvent network policies or security controls
+      intended to prevent/isolate access to those backends.
+  labels:
+    rbac.authorization.k8s.io/aggregate-to-edit: "true"
+  name: custom:aggregate-to-edit:endpoints # you can change this if you wish
+rules:
+  - apiGroups: [""]
+    resources: ["endpoints"]
+    verbs: ["create", "delete", "deletecollection", "patch", "update"]