The remaining sections are handled in the dumux course
and the examples.

[doc] Feature/doc dissolve handbook (partial)

See merge request !3580

[doxygen] Move input/output section from handbook to doxygen

See merge request !3579

[doc][doxygen] Feature/move physics to doxygen

See merge request !3578

Add handbook Chapter 2.3 Parameters to doxygen

See merge request !3568

[doxygen] Move spatial discretization documentation from handbook to doxygen

See merge request !3571

Cleanup/handbook mpfa without scv

See merge request !3566

[cleanup] Simplify and remove some code deprecated in the previous release

See merge request !3557