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/customresourcedefinition.md

@@ -0,0 +1,46 @@
+---
+title: CustomResourceDefinition
+id: CustomResourceDefinition
+date: 2018-04-12
+full_link: /zh/docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/
+short_description: >
+  通过定制化的代码给你的 Kubernetes API 服务器增加资源对象，而无需编译完整的定制 API 服务器。
+
+aka: 
+tags:
+- fundamental
+- operation
+- extension
+---
+
+<!--
+---
+title: CustomResourceDefinition
+id: CustomResourceDefinition
+date: 2018-04-12
+full_link: /docs/tasks/extend-kubernetes/custom-resources/custom-resource-definitions/
+  Custom code that defines a resource to add to your Kubernetes API server without building a complete custom server.
+
+aka: 
+tags:
+- fundamental
+- operation
+- extension
+---
+-->
+ 
+<!--
+ Custom code that defines a resource to add to your Kubernetes API server without building a complete custom server.
+-->
+
+ 通过定制化的代码给你的 Kubernetes API 服务器增加资源对象，而无需编译完整的定制 API 服务器。
+
+<!--more--> 
+
+<!--
+Custom Resource Definitions let you extend the Kubernetes API for your environment if the publicly supported API resources can't meet your needs. 
+-->
+
+当 Kubernetes 公开支持的 API 资源不能满足你的需要时，
+定制资源对象（Custom Resource Definitions）让你可以在你的环境上扩展 Kubernetes API。
+