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/kubeadm-join-phase.md

@@ -0,0 +1,101 @@
+---
+title: kubeadm join phase
+weight: 90
+---
+
+<!--
+title: kubeadm join phase
+weight: 90
+-->
+
+<!--
+`kubeadm join phase` enables you to invoke atomic steps of the join process.
+Hence, you can let kubeadm do some of the work and you can fill in the gaps
+if you wish to apply customization.
+-->
+`kubeadm join phase` 使你能够调用 `join` 过程的基本原子步骤。
+因此，如果希望执行自定义操作，可以让 kubeadm 做一些工作，然后由用户来补足剩余操作。
+
+<!--
+`kubeadm join phase` is consistent with the [kubeadm join workflow](/docs/reference/setup-tools/kubeadm/kubeadm-join/#join-workflow),
+and behind the scene both use the same code.
+-->
+`kubeadm join phase` 与
+[kubeadm join 工作流程](/zh/docs/reference/setup-tools/kubeadm/kubeadm-join/#join-workflow)
+一致，后台都使用相同的代码。
+
+## kubeadm join phase {#cmd-join-phase}
+
+{{< tabs name="tab-phase" >}}
+{{< tab name="phase" include="generated/kubeadm_join_phase.md" />}}
+{{< /tabs >}}
+
+## kubeadm join phase preflight {#cmd-join-phase-preflight}
+
+<!--
+Using this phase you can execute preflight checks on a joining node.
+-->
+使用此命令可以在即将加入集群的节点上执行启动前检查。
+
+{{< tabs name="tab-preflight" >}}
+{{< tab name="preflight" include="generated/kubeadm_join_phase_preflight.md" />}}
+{{< /tabs >}}
+
+## kubeadm join phase control-plane-prepare {#cmd-join-phase-control-plane-prepare}
+
+<!--
+Using this phase you can prepare a node for serving a control-plane.
+-->
+使用此阶段，你可以准备一个作为控制平面的节点。
+
+{{< tabs name="tab-control-plane-prepare" >}}
+{{< tab name="control-plane-prepare" include="generated/kubeadm_join_phase_control-plane-prepare.md" />}}
+{{< tab name="all" include="generated/kubeadm_join_phase_control-plane-prepare_all.md" />}}
+{{< tab name="download-certs" include="generated/kubeadm_join_phase_control-plane-prepare_download-certs.md" />}}
+{{< tab name="certs" include="generated/kubeadm_join_phase_control-plane-prepare_certs.md" />}}
+{{< tab name="kubeconfig" include="generated/kubeadm_join_phase_control-plane-prepare_kubeconfig.md" />}}
+{{< tab name="control-plane" include="generated/kubeadm_join_phase_control-plane-prepare_control-plane.md" />}}
+{{< /tabs >}}
+
+## kubeadm join phase kubelet-start {#cmd-join-phase-kubelet-start}
+
+<!--
+Using this phase you can write the kubelet settings, certificates and (re)start the kubelet.
+-->
+使用此阶段，你可以配置 kubelet 设置、证书和（重新）启动 kubelet。
+
+{{< tabs name="tab-kubelet-start" >}}
+{{< tab name="kubelet-start" include="generated/kubeadm_join_phase_kubelet-start.md" />}}
+{{< /tabs >}}
+
+## kubeadm join phase control-plane-join {#cmd-join-phase-control-plane-join}
+
+<!--
+Using this phase you can join a node as a control-plane instance.
+-->
+使用此阶段，你可以将节点作为控制平面实例加入。
+
+{{< tabs name="tab-control-plane-join" >}}
+{{< tab name="control-plane-join" include="generated/kubeadm_join_phase_control-plane-join.md" />}}
+{{< tab name="all" include="generated/kubeadm_join_phase_control-plane-join_all.md" />}}
+{{< tab name="etcd" include="generated/kubeadm_join_phase_control-plane-join_etcd.md" />}}
+{{< tab name="update-status" include="generated/kubeadm_join_phase_control-plane-join_update-status.md" />}}
+{{< tab name="mark-control-plane" include="generated/kubeadm_join_phase_control-plane-join_mark-control-plane.md" />}}
+{{< /tabs >}}
+
+## {{% 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 connect a node to the cluster
+* [kubeadm reset](/docs/reference/setup-tools/kubeadm/kubeadm-reset/) to revert any changes made to this host by `kubeadm init` or `kubeadm join`
+* [kubeadm alpha](/docs/reference/setup-tools/kubeadm/kubeadm-alpha/) to try experimental functionality
+-->
+* [kubeadm init](/zh/docs/reference/setup-tools/kubeadm/kubeadm-init/)
+  引导 Kubernetes 控制平面节点
+* [kubeadm join](/zh/docs/reference/setup-tools/kubeadm/kubeadm-join/)
+  将节点添加到集群
+* [kubeadm reset](/zh/docs/reference/setup-tools/kubeadm/kubeadm-reset/)
+  恢复通过 `kubeadm init` 或 `kubeadm join` 操作对主机所做的任何更改
+* [kubeadm alpha](/zh/docs/reference/setup-tools/kubeadm/kubeadm-alpha/)
+  尝试实验性功能