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-configmap.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: mysql
+  labels:
+    app: mysql
+data:
+  primary.cnf: |
+    # Apply this config only on the primary.
+    [mysqld]
+    log-bin
+    datadir=/var/lib/mysql/mysql
+  replica.cnf: |
+    # Apply this config only on replicas.
+    [mysqld]
+    super-read-only
+    datadir=/var/lib/mysql/mysql
+