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/pod-single-configmap-env-variable.yaml

@@ -0,0 +1,19 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: dapi-test-pod
+spec:
+  containers:
+    - name: test-container
+      image: k8s.gcr.io/busybox
+      command: [ "/bin/sh", "-c", "env" ]
+      env:
+        # 定义环境变量
+        - name: SPECIAL_LEVEL_KEY
+          valueFrom:
+            configMapKeyRef:
+              # ConfigMap 包含你要赋给 SPECIAL_LEVEL_KEY 的值
+              name: special-config
+              # 指定与取值相关的键名
+              key: special.how
+  restartPolicy: Never