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/cgroup.md

@@ -0,0 +1,40 @@
+---
+title: 控制组（cgroup）
+id: cgroup
+date: 2019-06-25
+full_link:
+short_description: >
+  一组具有可选资源隔离、审计和限制的 Linux 进程。
+
+aka:
+tags:
+- fundamental
+---
+
+<!--
+---
+title: cgroup (control group)
+id: cgroup
+date: 2019-06-25
+full_link:
+short_description: >
+  A group of Linux processes with optional resource isolation, accounting and limits.
+
+aka:
+tags:
+- fundamental
+---
+-->
+
+<!--
+A group of Linux processes with optional resource isolation, accounting and limits.
+-->
+一组具有可选资源隔离、审计和限制的 Linux 进程。
+
+<!--more--> 
+
+<!--
+cgroup is a Linux kernel feature that limits, accounts for, and isolates the resource usage (CPU, memory, disk I/O, network) for a collection of processes.
+-->
+
+Cgroup 是一个 Linux 内核特性，对一组进程的资源使用（CPU、内存、磁盘 I/O 和网络等）进行限制、审计和隔离。