Merge 'External Modules' and 'Extension Modules'

The difference between 'Extension Modules' and 'External Modules' is not very obvious, especially to novice users. While the former are all hosted on our gitlab instance, this technical difference does not justify another category. It seems that it's more about: For some extension modules we don't have a module box because no core developer is maintaining it and we lack information. Instead of introducing the new term 'External Module' it would IMO be more user friendly to just list those below the generated list on 'Extension Modules' and drop the 'External Modules' category.