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/containerd.md

@@ -0,0 +1,39 @@
+---
+title: containerd
+id: containerd
+date: 2019-05-14
+full_link: https://containerd.io/docs/
+short_description: >
+ 强调简单性、健壮性和可移植性的一种容器运行时
+aka:
+tags:
+- tool
+---
+
+<!--
+---
+title: containerd
+id: containerd
+date: 2019-05-14
+full_link: https://containerd.io/docs/
+short_description: >
+  A container runtime with an emphasis on simplicity, robustness and portability
+
+aka:
+tags:
+- tool
+---
+-->
+
+ 强调简单性、健壮性和可移植性的一种容器运行时
+
+<!--more-->
+
+<!--
+containerd is a {{< glossary_tooltip text="container" term_id="container" >}} runtime
+that runs as a daemon on Linux or Windows. containerd takes care of fetching and
+storing container images, executing containers, providing network access, and more.
+-->
+
+containerd 是一种{{< glossary_tooltip text="容器" term_id="container" >}}运行时，能在 Linux 或者 Windows 后台运行。
+containerd 能取回、存储容器镜像，执行容器实例，提供网络访问等。