Change link in documentation to point to new homepage of UG at Heidelberg
(cherry picked from commit 89549983ad5c9e94d311c14e86c355a56ed57c09)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
Reformat the doxygen short documentation to make it more useful
(cherry picked from commit fa66a96dbe52d95e3b007cb7f1bc2f0774499d9a)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>