Well the thing is that "modules" is more an artificial thing than an actual C++ term (maybe once we get C++ modules in C++22 (?) it may change).
I'd argue that what we have already is more useful than a big diagram that has everything on it.
If you want to see all the module related classes, you can pick a module in the
Modules tab.
The inheritance class diagram already shows you all the connection with clickable links to the other classes. There are no additional inheritances to be shown. Plus you can easily navigate to the module it's in and see other classes.
And finally if you run doxygen yourself, you can also get some additional diagrams, like an inheritance page that shows all the classes and their full dependencies.
Or call and caller graphs for each function, but that shows too much internal detail that you should not worry about as a API consumer.