why need config

Cocos website exists:

1. English: http://www.cocos2d-x.org/
2. 中文：http://www.cocos.com/

the gitbook be published twice, links are different on both. So add the config to adapt this.

how to change

Config of the master branch will be for cocos.com, if you need to change config to cocos2d-x.org, only need run:

cd to docs root dir
> cp -rf config/cocos2d-x.org/ ./