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/storage/projected-secret-downwardapi-configmap.yaml

@@ -0,0 +1,35 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: volume-test
+spec:
+  containers:
+  - name: container-test
+    image: busybox:1.28
+    volumeMounts:
+    - name: all-in-one
+      mountPath: "/projected-volume"
+      readOnly: true
+  volumes:
+  - name: all-in-one
+    projected:
+      sources:
+      - secret:
+          name: mysecret
+          items:
+            - key: username
+              path: my-group/my-username
+      - downwardAPI:
+          items:
+            - path: "labels"
+              fieldRef:
+                fieldPath: metadata.labels
+            - path: "cpu_limit"
+              resourceFieldRef:
+                containerName: container-test
+                resource: limits.cpu
+      - configMap:
+          name: myconfigmap
+          items:
+            - key: config
+              path: my-group/my-config