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/setup/production-environment/turnkey-solutions.md

@@ -0,0 +1,25 @@
+---
+title: Turnkey 云解决方案
+content_type: concept
+weight: 30
+---
+<!-- 
+---
+title: Turnkey Cloud Solutions
+content_type: concept
+weight: 30
+---
+-->
+<!-- overview -->
+
+<!-- 
+This page provides a list of Kubernetes certified solution providers. From each
+provider page, you can learn how to install and setup production
+ready clusters.
+-->
+本页列示 Kubernetes 认证解决方案供应商。
+在每一个供应商分页，你可以学习如何安装和设置生产就绪的集群。
+
+<!-- body -->
+
+{{< cncf-landscape helpers=true category="certified-kubernetes-hosted" >}}