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/example-ingress.yaml

@@ -0,0 +1,18 @@
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  name: example-ingress
+  annotations:
+    nginx.ingress.kubernetes.io/rewrite-target: /$1
+spec:
+  rules:
+    - host: hello-world.info
+      http:
+        paths:
+          - path: /
+            pathType: Prefix
+            backend:
+              service:
+                name: web
+                port:
+                  number: 8080