Reorganise the documentation

I think it would be good to reorganise the documentation and split it up into four parts:

*) A user manual (docbook)
*) Developer manual (docbook and jsdoc)
*) Howtos (either in docbook or jessyink-svg)
*) Showcase files (jessyink-svg)

The user manual should pretty much cover everything JessyInk can do. The developer docs should help people who want to improve JessyInk by providing information about the architecture, development stack and source code documentation. HowTo files should explain how certain commonly needed tasks can be achieved. The showcase files should present examples of what JessyInk can be used for.