Rework documentation structure

The approach in our tool documentation main pages are a bit different if you compare http://doc.qt.io/qt3dstudio/index.html and http://doc.qt.io/qtcreator/index.html. Also if you look at e.g. https://docs.blender.org/manual/en/dev/ which is a bit similar approach as in Qt Creator.

I think we should try to re-think the whole documentation structure and e.g. highlight a bit the topics that are a bit hidden in the Best Practices section.