A while ago I decided that there is one particular module in a Maven-based NetBeans Platform application that I want to document properly with Javadocs, since it contains an API that other developers will (hopefully) use in future. And so I set out to find where the overview.html and package.html files should go. It turns out that it is actually very simple – just follow the same structure as for any Maven project! There is a very useful description of the structure available here.
Lesson learned: If you don’t find any useful information about some aspect of building Maven-based NetBeans Module projects specifically, the answer might very well just be a standard, generic Maven one.