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/developer.md

@@ -0,0 +1,44 @@
+---
+title: 开发者（Developer）
+id: developer
+date: 2018-04-12
+full_link: 
+short_description: >
+  指的是：应用开发者、代码贡献者、或平台开发者。
+
+aka: 
+tags:
+- community
+- user-type
+---
+
+<!--
+---
+title: Developer (disambiguation)
+id: developer
+date: 2018-04-12
+full_link: 
+short_description: >
+  May refer to&#58; Application Developer, Code Contributor, or Platform Developer.
+
+aka: 
+tags:
+- community
+- user-type
+---
+-->
+
+ 指的是：{{< glossary_tooltip text="应用开发者" term_id="application-developer" >}}、
+ {{< glossary_tooltip text="代码贡献者" term_id="code-contributor" >}}、
+ 或{{< glossary_tooltip text="平台开发者" term_id="platform-developer" >}}。
+
+<!--more--> 
+
+<!--
+This overloaded term may have different meanings depending on the context
+-->
+
+根据上下文的不同，“开发者”这个被多处使用的词条会有不同的含义。
+
+
+