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/tutorials/kubernetes-basics/create-cluster/_index.md

@@ -0,0 +1,10 @@
+---
+title: 创建集群
+weight: 10
+---
+
+<!--
+Learn about Kubernetes {{< glossary_tooltip text="cluster" term_id="cluster" length="all" >}} and create a simple cluster using Minikube.
+-->
+了解 Kubernetes {{<glossary_tooltip text="集群" term_id="cluster" length="all" >}}并使用 Minikube
+创建一个简单的集群。