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/manifest.md

@@ -0,0 +1,32 @@
+---
+title: 清单（Manifest）
+id: manifest
+date: 2019-06-28
+short_description: >
+  一个或多个 Kubernetes API 对象的序列化规范。
+
+aka:
+tags:
+- fundamental
+---
+ JSON 或 YAML 格式的 Kubernetes API 对象规范。
+
+<!--
+title: Manifest
+id: manifest
+date: 2019-06-28
+short_description: >
+  A serialized specification of one or more Kubernetes API objects.
+
+aka:
+tags:
+- fundamental
+ Specification of a Kubernetes API object in JSON or YAML format.
+-->
+
+<!--more-->
+
+<!-- 
+A manifest specifies the desired state of an object that Kubernetes will maintain when you apply the manifest. Each configuration file can contain multiple manifests.
+ -->
+清单指定了在应用该清单时 kubernetes 将维护的对象的期望状态。每个配置文件可包含多个清单。