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/security/kind-with-namespace-level-baseline-pod-security.sh

@@ -0,0 +1,28 @@
+#!/bin/sh
+# 直到 v1.23 发布，kind 节点镜像需要从 k/k master 分支构建
+# 参考：https://kind.sigs.k8s.io/docs/user/quick-start/#building-images
+kind create cluster --name psa-ns-level --image kindest/node:v1.23.0
+kubectl cluster-info --context kind-psa-ns-level
+# 等待 15 秒（任意）ServiceAccount 准入控制器可用
+sleep 15
+kubectl create ns example
+kubectl label --overwrite ns example \
+  pod-security.kubernetes.io/enforce=baseline \
+  pod-security.kubernetes.io/enforce-version=latest \
+  pod-security.kubernetes.io/warn=restricted \
+  pod-security.kubernetes.io/warn-version=latest \
+  pod-security.kubernetes.io/audit=restricted \
+  pod-security.kubernetes.io/audit-version=latest
+cat <<EOF > /tmp/pss/nginx-pod.yaml
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx
+spec:
+  containers:
+    - image: nginx
+      name: nginx
+      ports:
+        - containerPort: 80
+EOF
+kubectl apply -n example -f /tmp/pss/nginx-pod.yaml