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/includes/user-guide-content-moved.md

@@ -0,0 +1,8 @@
+<!--
+The topics in the [User Guide](/docs/user-guide/) section of the Kubernetes docs
+are being moved to the [Tasks](/docs/tasks/), [Tutorials](/docs/tutorials/), and
+[Concepts](/docs/concepts) sections. The content in this topic has moved to:
+-->
+Kubernetes文档中[用户指南](/zh/docs/user-guide/)部分中的主题被移动到
+[任务](/zh/docs/tasks/)、[教程](/zh/docs/tutorials/)和[概念](/zh/docs/concepts)节。
+本主题内容已移至: