[cleanup] Remove some out-commented code
(cherry picked from commit f6bf5675f58d830bb7f15d72c4c21989b890ada4)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[documentation] Fix minor errors
(cherry picked from commit 58572521755d0300f7171be5109b9e87a0b38ebc)

Signed-off-by: Christoph Grüninger <gruenich@dune-project.org>
[release] Update list of copyright holders until Dune 2.3.
(cherry picked from commit 57aa8f4fdd7760eb48790015b71b66509ce454a3)

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