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/glossary/cluster-operations.md

@@ -0,0 +1,42 @@
+---
+title: 集群操作（Cluster Operations）
+id: cluster-operations
+date: 2019-05-12
+full_link:
+short_description: >
+ 管理 Kubernetes 集群所涉及的相关工作。
+
+aka:
+tags:
+- operations
+---
+
+<!--
+---
+title: Cluster Operations
+id: cluster-operations
+date: 2019-05-12
+full_link:
+short_description: >
+ The work involved in managing a Kubernetes cluster.
+
+aka:
+tags:
+- operation
+---
+-->
+
+<!--
+ The work involved in managing a Kubernetes cluster: managing
+day-to-day operations, and co-ordinating upgrades.
+-->
+Kubernetes 管理相关工作包括：日常管理操作和协调升级。
+
+<!--
+ Examples of cluster operations work include: deploying new Nodes to
+scale the cluster; performing software upgrades; implementing security
+controls; adding or removing storage; configuring cluster networking;
+managing cluster-wide observability; and responding to events.
+-->															  
+集群操作工作的示例包括：部署新节点来扩容集群；执行软件升级；实施安全控制；
+添加或删除存储；配置集群网络；管理集群范围的可观测性；响应集群事件。