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/reference/glossary/code-contributor.md

@@ -0,0 +1,43 @@
+---
+title: 代码贡献者（Code Contributor）
+id: code-contributor
+date: 2018-04-12
+full_link: /docs/community/devel/
+short_description: >
+  为 Kubernetes 开源代码库开发并贡献代码的人。
+
+aka: 
+tags:
+- community
+- user-type
+---
+
+<!--
+---
+title: Code Contributor
+id: code-contributor
+date: 2018-04-12
+full_link: /docs/community/devel/
+short_description: >
+  A person who develops and contributes code to the Kubernetes open source codebase.
+
+aka: 
+tags:
+- community
+- user-type
+---
+-->
+
+<!--
+ A person who develops and contributes code to the Kubernetes open source codebase.
+-->
+
+ 为 Kubernetes 开源代码库开发并贡献代码的人。
+
+<!--more--> 
+
+<!--
+They are also an active {{< glossary_tooltip text="community member" term_id="member" >}} who participates in one or more {{< glossary_tooltip text="Special Interest Groups (SIGs)" term_id="sig" >}}.
+-->
+
+代码贡献者也是加入一个或多个 {{< glossary_tooltip text="特别兴趣小组 (SIGs)" term_id="sig" >}} 的活跃的 {{< glossary_tooltip text="社区成员" term_id="member" >}}。