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/annotation.md

@@ -0,0 +1,46 @@
+---
+title: 注解（Annotation）
+id: annotation
+date: 2018-04-12
+full_link: /zh/docs/concepts/overview/working-with-objects/annotations/
+short_description: >
+  注解是以键值对的形式给资源对象附加随机的无法标识的元数据。
+
+aka: 
+tags:
+- fundamental
+---
+
+<!--
+---
+title: Annotation
+id: annotation
+date: 2018-04-12
+full_link: /docs/concepts/overview/working-with-objects/annotations
+short_description: >
+  A key-value pair that is used to attach arbitrary non-identifying metadata to objects.
+
+aka: 
+tags:
+- fundamental
+---
+-->
+
+
+
+<!--
+ A key-value pair that is used to attach arbitrary non-identifying metadata to objects.
+-->
+
+ 注解是以键值对的形式给资源对象附加随机的无法标识的元数据。
+
+<!--more--> 
+
+<!--
+The metadata in an annotation can be small or large, structured or unstructured, and can include characters not permitted by {{< glossary_tooltip text="labels" term_id="label" >}}. Clients such as tools and libraries can retrieve this metadata.
+-->
+
+注解中的元数据可大可小，可以是结构化的也可以是非结构化的，
+并且能包含{{< glossary_tooltip text="标签" term_id="label" >}}不允许使用的字符。
+像工具和软件库这样的客户端可以检索这些元数据。
+