Renamed hooks cookbook article
Restructured API folder

API = Application Programming Interface
in my opinion, hooks and callbacks are part of the developer API
Merge branch '2.11' into 3.0
Merge branch '3.0' into 3.1
Merge branch '3.1' into 3.1-ja.
Do not use special characters in the file name
Merge branch '3.1' into 3.1-ja.