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/cidr.md

@@ -0,0 +1,43 @@
+---
+title: CIDR
+id: cidr
+date: 2019-11-12
+full_link: 
+short_description: >
+  CIDR 是一种描述 IP 地址块的符号，被广泛使用于各种网络配置中。
+
+aka:
+tags:
+- networking
+---
+
+<!--
+---
+title: CIDR
+id: cidr
+date: 2019-11-12
+full_link: 
+short_description: >
+  CIDR is a notation for describing blocks of IP addresses and is used heavily in various networking configurations.
+
+aka:
+tags:
+- networking
+---
+-->
+
+<!--
+CIDR (Classless Inter-Domain Routing) is a notation for describing blocks of IP addresses and is used heavily in various networking configurations.
+-->
+CIDR (无类域间路由) 是一种描述 IP 地址块的符号，被广泛使用于各种网络配置中。
+
+
+<!--more-->
+
+<!--
+In the context of Kubernetes, each {{< glossary_tooltip text="Node" term_id="node" >}} is assigned a range of IP addresses through the start address and a subnet mask using CIDR. This allows Nodes to assign each {{< glossary_tooltip text="Pod" term_id="pod" >}} a unique IP address. Although originally a concept for IPv4, CIDR has also been expanded to include IPv6. 
+-->
+在 Kubernetes 的上下文中，每个{{< glossary_tooltip text="节点" term_id="node" >}}
+以 CIDR 形式（含起始地址和子网掩码）获得一个 IP 地址段，
+从而能够为每个 {{< glossary_tooltip text="Pod" term_id="pod" >}} 分配一个独一无二的 IP 地址。
+虽然其概念最初源自 IPv4，CIDR 已经被扩展为涵盖 IPv6。