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/sig.md

@@ -0,0 +1,51 @@
+---
+title: 特别兴趣小组（SIG）
+id: sig
+date: 2018-04-12
+full_link: https://github.com/kubernetes/community/blob/master/sig-list.md#master-sig-list
+short_description: >
+  共同管理大范畴 Kubernetes 开源项目中某组件或方面的一组社区成员。
+
+aka: 
+tags:
+- community
+---
+
+<!--
+---
+title: SIG (special interest group)
+id: sig
+date: 2018-04-12
+full_link: https://github.com/kubernetes/community/blob/master/sig-list.md#master-sig-list
+short_description: >
+  Community members who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project.
+
+aka: 
+tags:
+- community
+---
+-->
+
+
+<!--
+ {{< glossary_tooltip text="Community members" term_id="member" >}} who collectively manage an ongoing piece or aspect of the larger Kubernetes open source project.
+-->
+
+共同管理大范畴 Kubernetes 开源项目中某组件或方面的一组{{< glossary_tooltip text="社区成员" term_id="member" >}}。 
+
+<!--more--> 
+
+<!--
+Members within a SIG have a shared interest in advancing a specific area, such as architecture, API machinery, or documentation.
+SIGs must follow the SIG [governance guidelines](https://github.com/kubernetes/community/blob/master/committee-steering/governance/sig-governance.md), but can have their own contribution policy and channels of communication.
+
+For more information, see the [kubernetes/community](https://github.com/kubernetes/community) repo and the current list of [SIGs and Working Groups](https://github.com/kubernetes/community/blob/master/sig-list.md).
+-->
+
+SIG 中的成员对推进某个领域（如体系结构、API 机制构件或者文档）具有相同的兴趣。
+SIGs 必须遵从 [governance guidelines](https://github.com/kubernetes/community/blob/master/committee-steering/governance/sig-governance.md) 的规定，
+不过可以有自己的贡献策略以及通信渠道（方式）。
+
+更多的详细信息可参阅 [kubernetes/community](https://github.com/kubernetes/community) 仓库以及
+[SIGs 和工作组（Working Groups）](https://github.com/kubernetes/community/blob/master/sig-list.md)的最新列表。
+