Reorganize the site and write howto tutorials

Right now we have reference documentation in http://cargo.codehaus.org/Features but we don't have howto tutorials for all APIs. We do have those for the m2 plugin though. We need to generalize this. BTW we also need to generalize the reference documentation for all APIs...

It would be nice that those tutorials do not repeat too much what's in the reference documentation. This could be achieved by:

• using confluence includes so that both howto and reference point to them
• linking a lot to the reference documentation in the howo tutorials