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/api-group.md

@@ -0,0 +1,50 @@
+---
+title: API Group
+id: api-group
+date: 2019-09-02
+full_link: /zh/docs/concepts/overview/kubernetes-api/#api-groups-and-versioning
+short_description: >
+  Kubernetes API 中的一组相关路径
+
+aka:
+tags:
+- fundamental
+- architecture
+---
+
+<!-- 
+---
+title: API Group
+id: api-group
+date: 2019-09-02
+full_link: /docs/concepts/overview/kubernetes-api/#api-groups-and-versioning
+short_description: >
+  A set of related paths in the Kubernetes API.
+
+aka:
+tags:
+- fundamental
+- architecture
+---										  
+-->
+
+
+<!-- 
+A set of related paths in Kubernetes API.  
+-->
+Kubernetes API 中的一组相关路径。
+
+<!--more-->																																																																											  
+
+<!-- 
+You can enable or disable each API group by changing the configuration of your API server. You can also disable or enable paths to specific resources. API group makes it easier to extend the Kubernetes API. The API group is specified in a REST path and in the `apiVersion` field of a serialized object. 
+-->
+通过更改 API server 的配置，可以启用或禁用每个 API Group。
+你还可以禁用或启用指向特定资源的路径。
+API group 使扩展 Kubernetes API 更加的容易。
+API group 在 REST 路径和序列化对象的 `apiVersion` 字段中指定。
+
+<!-- 
+* Read [API Group](/docs/concepts/overview/kubernetes-api/#api-groups-and-versioning) for more information. 
+-->
+* 阅读 [API Group](/zh/docs/concepts/overview/kubernetes-api/#api-groups-and-versioning) 了解更多信息。