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/service/networking/ingress-resource-backend.yaml

@@ -0,0 +1,20 @@
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: ingress-resource-backend
+spec:
+  defaultBackend:
+    resource:
+      apiGroup: k8s.example.com
+      kind: StorageBucket
+      name: static-assets
+  rules:
+    - http:
+        paths:
+          - path: /icons
+            pathType: ImplementationSpecific
+            backend:
+              resource:
+                apiGroup: k8s.example.com
+                kind: StorageBucket
+                name: icon-assets