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/garbage-collection.md

@@ -0,0 +1,49 @@
+---
+title: 垃圾收集
+id: garbage-collection
+date: 2021-07-07
+full_link: /zh/docs/concepts/workloads/controllers/garbage-collection/
+short_description: >
+  Kubernetes 用于清理集群资源的各种机制的统称。
+
+aka: 
+tags:
+- fundamental
+- operation
+---
+<!-- 
+title: Garbage Collection
+id: garbage-collection
+date: 2021-07-07
+full_link: /docs/concepts/workloads/controllers/garbage-collection/
+short_description: >
+  A collective term for the various mechanisms Kubernetes uses to clean up cluster
+  resources.
+
+aka: 
+tags:
+- fundamental
+- operation
+-->
+
+<!-- 
+ Garbage collection is a collective term for the various mechanisms Kubernetes uses to clean up
+ cluster resources. 
+-->
+垃圾收集是 Kubernetes 用于清理集群资源的各种机制的统称。
+
+<!--more-->
+
+<!-- 
+Kubernetes uses garbage collection to clean up resources like [unused containers and images](/docs/concepts/workloads/controllers/garbage-collection/#containers-images),
+[failed Pods](/docs/concepts/workloads/pods/pod-lifecycle/#pod-garbage-collection),
+[objects owned by the targeted resource](/docs/concepts/overview/working-with-objects/owners-dependents/),
+[completed Jobs](/docs/concepts/workloads/controllers/ttlafterfinished/), and resources
+that have expired or failed.
+-->
+Kubernetes 使用垃圾收集机制来清理资源，例如：
+[未使用的容器和镜像](/zh/docs/concepts/workloads/controllers/garbage-collection/#containers-images)、
+[失败的 Pod](/zh/docs/concepts/workloads/pods/pod-lifecycle/#pod-garbage-collection)、
+[目标资源拥有的对象](/zh/docs/concepts/overview/working-with-objects/owners-dependents/)、
+[已完成的 Job](/zh/docs/concepts/workloads/controllers/ttlafterfinished/)、
+过期或出错的资源。