fix **** in doxygen comments

to make them stand out more prominently, some entity comments
where started with a line of starts. Unfortunately, doxygen
(and javadoc) only recogise comments which are started exactly
with /**

This caused quite some comments to be ignored by doxygen.
Credits to Hendrik Boom for spotting this problem!

A workaround is to end the line of stars with *//**
DOC: reorganise the Doxygen configuration and structure

- 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