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/endpoint.md

@@ -0,0 +1,38 @@
+---
+title: 端点（Endpoints）
+id: endpoints
+date: 2020-04-23
+full_link: 
+short_description: >
+  端点负责记录与服务（Service）的选择器相匹配的 Pods 的 IP 地址。
+
+aka:
+tags:
+- networking
+---
+ 端点负责记录与服务的{{< glossary_tooltip text="选择器" term_id="selector" >}}相匹配的 Pods 的 IP 地址。
+
+<!--
+title: Endpoints
+id: endpoints
+date: 2020-04-23
+full_link: 
+short_description: >
+  Endpoints track the IP addresses of Pods with matching Service selectors.
+
+aka:
+tags:
+- networking
+   
+ Endpoints track the IP addresses of Pods with matching  {{< glossary_tooltip text="selectors" term_id="selector" >}}.
+-->
+
+<!--more-->
+<!--
+Endpoints can be configured manually for {{< glossary_tooltip text="Services" term_id="service" >}} without selectors specified.
+-->
+端点可以手动配置到{{< glossary_tooltip text="服务（Service）" term_id="service" >}}上，而不必指定选择器标识。
+<!--
+The {{< glossary_tooltip text="EndpointSlice" term_id="endpoint-slice" >}} resource provides a scalable and extensible alternative to Endpoints.
+-->
+{{< glossary_tooltip text="EndpointSlice" term_id="endpoint-slice" >}}提供了一种可伸缩、可扩展的替代方案。