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/docs/contribute/generate-ref-docs/_index.md

@@ -0,0 +1,26 @@
+---
+title: 参考文档概述
+main_menu: true
+weight: 80
+---
+
+<!--
+title: Reference docs overview
+main_menu: true
+weight: 80
+-->
+
+<!--
+The topics in this section document how to generate the Kubernetes
+reference guides.
+
+To build the reference documentation, see the following guide:
+
+* [Generating Reference Documentation Quickstart](/docs/contribute/generate-ref-docs/quickstart/)
+-->
+
+本节的主题是描述如何生成 Kubernetes 参考指南。
+要生成参考文档，请参考下面的指南：
+
+* [生成参考文档快速入门](/zh/docs/contribute/generate-ref-docs/quickstart/)
+