BuilderDoxygen and BuilderGCH are external plug-ins,
not developed in this project and probably unmaintained.
TODO: decide how to fix or replace them...
- upgrade the configuration to a current version
- provide a frontpage with cross-links to other documentation
- define a set of modules; relevant classes and files can be
added to these, to create a exploration path for new readers
- fix a lot of errors in documentation comments
- use a custom configuration for the documentation pages
- tweak the navigation, the sections and further arrangements
