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/contributor.md

@@ -0,0 +1,39 @@
+---
+title: 贡献者（Contributor）
+id: contributor
+date: 2018-04-12
+full_link: 
+short_description: >
+  通过贡献代码、文档或者投入时间等方式来帮助 Kubernetes 项目或社区的人。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+title: Contributor
+id: contributor
+date: 2018-04-12
+full_link: 
+short_description: >
+  Someone who donates code, documentation, or their time to help the Kubernetes project or community.
+
+aka: 
+tags:
+- community
+-->
+
+<!--
+ Someone who donates code, documentation, or their time to help the Kubernetes project or community.
+-->
+
+通过贡献代码、文档或者投入时间等方式来帮助 Kubernetes 项目或社区的人。
+
+<!--more--> 
+<!--
+Contributions include pull requests (PRs), issues, feedback, {{< glossary_tooltip text="special interest groups (SIG)" term_id="sig" >}} participation, or organizing community events.
+-->
+
+贡献形式包括提交拉取请求（PRs）、问题报告（Issues）、反馈、参与{{< glossary_tooltip text="特别兴趣小组（SIG）" term_id="sig" >}}或者组织社区活动等等。
+