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/includes/default-storage-class-prereqs.md

@@ -0,0 +1,12 @@
+你需要有一个带有默认 [StorageClass](/zh/docs/concepts/storage/storage-classes/)的
+[动态 PersistentVolume 供应程序](/zh/docs/concepts/storage/dynamic-provisioning/)，
+或者自己[静态的提供 PersistentVolume](/zh/docs/concepts/storage/persistent-volumes/#provisioning)
+来满足这里使用的 [PersistentVolumeClaim](/zh/docs/concepts/storage/persistent-volumes/#persistentvolumeclaims)。
+
+<!--
+You need to either have a [dynamic PersistentVolume provisioner](/docs/concepts/storage/dynamic-provisioning/) with a default
+[StorageClass](/docs/concepts/storage/storage-classes/),
+or [statically provision PersistentVolumes](/docs/concepts/storage/persistent-volumes/#provisioning)
+yourself to satisfy the [PersistentVolumeClaims](/docs/concepts/storage/persistent-volumes/#persistentvolumeclaims)
+used here.
+-->