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/style/_index.md

@@ -0,0 +1,19 @@
+---
+title: 文档样式概述
+main_menu: true
+weight: 80
+---
+
+<!--
+title: Documentation style overview
+main_menu: true
+weight: 80
+-->
+
+<!--
+The topics in this section provide guidance on writing style, content formatting
+and organization, and using Hugo customizations specific to Kubernetes
+documentation.
+-->
+本节的主题是提供有关写作风格、内容格式和组织以及如何使用
+特定于 Kubernetes 文档的 Hugo 定制代码的指导。