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/setup-tools/kubeadm/_index.md

@@ -0,0 +1,79 @@
+---
+title: "Kubeadm"
+weight: 10
+no_list: true
+content_type: concept
+card:
+  name: reference
+  weight: 40
+---
+
+<img src="/images/kubeadm-stacked-color.png" align="right" width="150px">
+
+<!-- 
+Kubeadm is a tool built to provide `kubeadm init` and `kubeadm join` as best-practice "fast paths" for creating Kubernetes clusters.
+ -->
+Kubeadm 是一个提供了 `kubeadm init` 和 `kubeadm join` 的工具，
+作为创建 Kubernetes 集群的 “快捷途径” 的最佳实践。
+
+<!-- 
+kubeadm performs the actions necessary to get a minimum viable cluster up and running. By design, it cares only about bootstrapping,
+not about provisioning machines. Likewise, installing various nice-to-have addons, like the Kubernetes Dashboard, monitoring solutions, and cloud-specific addons, is not in scope.
+ -->
+kubeadm 通过执行必要的操作来启动和运行最小可用集群。
+按照设计，它只关注启动引导，而非配置机器。同样的，
+安装各种 “锦上添花” 的扩展，例如 Kubernetes Dashboard、
+监控方案、以及特定云平台的扩展，都不在讨论范围内。
+
+<!-- 
+Instead, we expect higher-level and more tailored tooling to be built on top of kubeadm, and ideally, using kubeadm as the basis of all deployments will make it easier to create conformant clusters.
+ -->
+相反，我们希望在 kubeadm 之上构建更高级别以及更加合规的工具，
+理想情况下，使用 kubeadm 作为所有部署工作的基准将会更加易于创建一致性集群。
+
+<!-- 
+## How to install
+ -->
+## 如何安装
+
+<!-- 
+To install kubeadm, see the [installation guide](/docs/setup/production-environment/tools/kubeadm/install-kubeadm).
+-->
+要安装 kubeadm, 请查阅
+[安装指南](/zh/docs/setup/production-environment/tools/kubeadm/install-kubeadm/).
+
+## {{% heading "whatsnext" %}}
+
+<!-- 
+* [kubeadm init](/docs/reference/setup-tools/kubeadm/kubeadm-init) to bootstrap a Kubernetes control-plane node
+* [kubeadm join](/docs/reference/setup-tools/kubeadm/kubeadm-join) to bootstrap a Kubernetes worker node and join it to the cluster
+* [kubeadm upgrade](/docs/reference/setup-tools/kubeadm/kubeadm-upgrade) to upgrade a Kubernetes cluster to a newer version
+* [kubeadm config](/docs/reference/setup-tools/kubeadm/kubeadm-config) if you initialized your cluster using kubeadm v1.7.x or lower, to configure your cluster for `kubeadm upgrade`
+* [kubeadm token](/docs/reference/setup-tools/kubeadm/kubeadm-token) to manage tokens for `kubeadm join`
+* [kubeadm reset](/docs/reference/setup-tools/kubeadm/kubeadm-reset) to revert any changes made to this host by `kubeadm init` or `kubeadm join`
+* [kubeadm certs](/docs/reference/setup-tools/kubeadm/kubeadm-certs) to manage Kubernetes certificates
+* [kubeadm kubeconfig](/docs/reference/setup-tools/kubeadm/kubeadm-kubeconfig) to manage kubeconfig files
+* [kubeadm version](/docs/reference/setup-tools/kubeadm/kubeadm-version) to print the kubeadm version
+* [kubeadm alpha](/docs/reference/setup-tools/kubeadm/kubeadm-alpha) to preview a set of features made available for gathering feedback from the community
+ -->
+* [kubeadm init](/zh/docs/reference/setup-tools/kubeadm/kubeadm-init)
+  用于搭建控制平面节点
+* [kubeadm join](/zh/docs/reference/setup-tools/kubeadm/kubeadm-join)
+  用于搭建工作节点并将其加入到集群中
+* [kubeadm upgrade](/zh/docs/reference/setup-tools/kubeadm/kubeadm-upgrade)
+  用于升级 Kubernetes 集群到新版本
+* [kubeadm config](/zh/docs/reference/setup-tools/kubeadm/kubeadm-config)
+  如果你使用了 v1.7.x 或更低版本的 kubeadm 版本初始化你的集群，则使用
+  `kubeadm upgrade` 来配置你的集群
+* [kubeadm token](/zh/docs/reference/setup-tools/kubeadm/kubeadm-token)
+  用于管理 `kubeadm join` 使用的令牌
+* [kubeadm reset](/zh/docs/reference/setup-tools/kubeadm/kubeadm-reset)
+  用于恢复通过 `kubeadm init` 或者 `kubeadm join` 命令对节点进行的任何变更
+* [kubeadm certs](/zh/docs/reference/setup-tools/kubeadm/kubeadm-certs)
+  用于管理 Kubernetes 证书
+* [kubeadm kubeconfig](/docs/reference/setup-tools/kubeadm/kubeadm-kubeconfig)
+  用于管理 kubeconfig 文件
+* [kubeadm version](/zh/docs/reference/setup-tools/kubeadm/kubeadm-version)
+  用于打印 kubeadm 的版本信息
+* [kubeadm alpha](/zh/docs/reference/setup-tools/kubeadm/kubeadm-alpha)
+  用于预览一组可用于收集社区反馈的特性