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/nginx-service.yaml

@@ -0,0 +1,16 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: nginx-service
+spec:
+  ports:
+  - port: 8000 # the port that this service should serve on
+    # the container on each pod to connect to, can be a name
+    # (e.g. 'www') or a number (e.g. 80)
+    targetPort: 80
+    protocol: TCP
+  # just like the selector in the deployment,
+  # but this time it identifies the set of pods to load balance
+  # traffic to.
+  selector:
+    app: nginx