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/cla.md

@@ -0,0 +1,41 @@
+---
+title: 贡献者许可协议（CLA）
+id: cla
+date: 2018-04-12
+full_link: https://github.com/kubernetes/community/blob/master/CLA.md
+short_description: >
+  贡献者对他们在开源项目中所贡献的代码的授权许可条款。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: CLA (Contributor License Agreement)
+id: cla
+date: 2018-04-12
+full_link: https://github.com/kubernetes/community/blob/master/CLA.md
+short_description: >
+  Terms under which a contributor grants a license to an open source project for their contributions.
+
+aka: 
+tags:
+- community
+---
+-->
+
+<!--
+ Terms under which a {{< glossary_tooltip text="contributor" term_id="contributor" >}} grants a license to an open source project for their contributions.
+-->
+ {{< glossary_tooltip text="贡献者" term_id="contributor" >}}对他们在开源项目中所贡献的代码的授权许可条款。
+
+<!--more--> 
+
+<!--
+CLAs help resolve legal disputes involving contributed material and intellectual property (IP).
+-->
+
+CLA 对解决贡献者在开源社区所贡献的资料和智力资产（IP）导致的法律纠纷很有帮助。
+