Is there any good (and simple) document which explain step by step how to customize the Ribbon interface (Adding/Modifying) panels etc.?
The Revit Interface is pretty locked down. As far as I know, you can only add tools to the Add-In tab. That's done by installing into the REVIT.INI file in the program directory.
You can do some basic modification such as tearing panels off and moving them to a second screen etc. But that's about it. To play devils advocate for a second, some BIM managers that I know actually like this standardise interface as it means the same tools are available in the same place to everybody which makes for more consistant learning and helps in documentation.