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/application/mysql/mysql-services.yaml

@@ -0,0 +1,29 @@
+# Headless service for stable DNS entries of StatefulSet members.
+apiVersion: v1
+kind: Service
+metadata:
+  name: mysql
+  labels:
+    app: mysql
+spec:
+  ports:
+  - name: mysql
+    port: 3306
+  clusterIP: None
+  selector:
+    app: mysql
+---
+# Client service for connecting to any MySQL instance for reads.
+# For writes, you must instead connect to the primary: mysql-0.mysql.
+apiVersion: v1
+kind: Service
+metadata:
+  name: mysql-read
+  labels:
+    app: mysql
+spec:
+  ports:
+  - name: mysql
+    port: 3306
+  selector:
+    app: mysql