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/README.md

@@ -0,0 +1,17 @@
+<!--
+To run the tests for a localization, use the following command:
+-->
+要运行本地化测试，请使用以下命令:
+
+```
+go test k8s.io/website/content/<lang>/examples
+```
+
+<!--
+where `<lang>` is the two character representation of a language. For example:
+-->
+其中 `<lang>` 是用两个字符表示一种语言。例如:
+
+```
+go test k8s.io/website/content/en/examples
+```