[zh_CN] rename zh to zh_CN for Simplified Chinese, leaving zh_TW for Traditional Chinese (ref: http://sphinx-doc.org/config.html#confval-language)
[zh] fix sphinx build warning
[zh_CN] change folder back to zh, but keep language as zh_CN
Merge pull request #1745 from blackpuppy/zh

[zh_CN] rename zh to zh_CN