The process to generate the docs now uses the full Doxyfile as given
by `doxygen -u Doxyfile.in` with a specific doxygen version, in this
commit version 1.8.14.
There's only one addition: 'HTML_COLORSTYLE = TOGGLE' which has been
added to doxygen in version 1.9.5. We're using it now to give the user
the choice to select a "dark mode" or "light mode" theme if the docs
are generated with doxygen 1.9.5 or later.
Unknown doxygen tags are now filtered silently and logged in files
documentation/Doxyfile_error.log and documentation/Doxybook_error.log,
respectively.
