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/includes/task-tutorial-prereqs.md

@@ -0,0 +1,14 @@
+你必须拥有一个 Kubernetes 的集群，同时你的 Kubernetes 集群必须带有 kubectl 命令行工具。
+建议在至少有两个节点的集群上运行本教程，且这些节点不作为控制平面主机。
+如果你还没有集群，你可以通过 [Minikube](https://minikube.sigs.k8s.io/docs/tutorials/multi_node/) 
+构建一个你自己的集群，或者你可以使用下面任意一个 Kubernetes 工具构建：
+<!--
+You need to have a Kubernetes cluster, and the kubectl command-line tool must
+be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a
+cluster, you can create one by using
+[minikube](https://minikube.sigs.k8s.io/docs/tutorials/multi_node/)
+or you can use one of these Kubernetes playgrounds:
+-->
+
+* [Katacoda](https://www.katacoda.com/courses/kubernetes/playground)
+* [玩转 Kubernetes](http://labs.play-with-k8s.com/)