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/federation-deprecation-warning-note.md

@@ -0,0 +1,8 @@
+<!--
+Use of `Federation v1` is strongly discouraged. `Federation V1` never achieved GA status and is no longer under active development. Documentation is for historical purposes only.
+
+For more information, see the intended replacement, [Kubernetes Federation v2](https://github.com/kubernetes-sigs/federation-v2).
+-->
+强烈建议不要使用`联邦 v1 版本`，`联邦 v1 版本`从未达到 GA 状态，且不再处于积极开发阶段。文档仅作为历史参考。
+
+有关更多信息，请参阅预期的替代品 [Kubernetes 联邦 v2 版本](https://github.com/kubernetes-sigs/federation-v2)。