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/blog/_posts/2015-03-00-Kubernetes-Gathering-Videos.md

@@ -0,0 +1,26 @@
+---
+
+title: " Kubernetes 采集视频 "
+date: 2015-03-23
+slug: kubernetes-gathering-videos
+---
+
+<!--
+---
+
+title: " Kubernetes Gathering Videos "
+date: 2015-03-23
+slug: kubernetes-gathering-videos
+url: /zh/blog/2015/03/Kubernetes-Gathering-Videos
+---
+-->
+
+<!--
+If you missed the Kubernetes Gathering in SF last month, fear not! Here are the videos from the evening presentations organized into a playlist on YouTube
+
+[![Kubernetes Gathering](https://img.youtube.com/vi/q8lGZCKktYo/0.jpg)](https://www.youtube.com/playlist?list=PL69nYSiGNLP2FBVvSLHpJE8_6hRHW8Kxe)
+-->
+
+如果你错过了上个月在旧金山举行的 Kubernetes 大会，不要害怕!以下是在 YouTube 上组织成播放列表的晚间演示文稿中的视频。
+
+[![Kubernetes Gathering](https://img.youtube.com/vi/q8lGZCKktYo/0.jpg)](https://www.youtube.com/playlist?list=PL69nYSiGNLP2FBVvSLHpJE8_6hRHW8Kxe)