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/addons.md

@@ -0,0 +1,40 @@
+---
+title: 附加组件（Add-ons）
+id: addons
+date: 2019-12-15
+full_link: /zh/docs/concepts/cluster-administration/addons/
+short_description: >
+  扩展 Kubernetes 功能的资源。
+
+aka:
+tags:
+- tool
+---
+<!--
+---
+title: Add-ons
+id: addons
+date: 2019-12-15
+full_link: /docs/concepts/cluster-administration/addons/
+short_description: >
+  Resources that extend the functionality of Kubernetes.
+
+aka:
+tags:
+- tool
+---
+-->
+
+
+<!--
+  Resources that extend the functionality of Kubernetes.
+-->
+  扩展 Kubernetes 功能的资源。
+
+
+<!--more-->
+
+<!-- 
+[Installing addons](/docs/concepts/cluster-administration/addons/) explains more about using add-ons with your cluster, and lists some popular add-ons.
+-->
+[安装附加组件](/zh/docs/concepts/cluster-administration/addons/) 阐释了更多关于如何在集群内使用附加组件，并列出了一些流行的附加组件。