Handbook for release 3.5

Todos:

• Update Dumux-version from 3.4 to 3.5
• Introduce a macro for the Dune version and update Dune-versions from 2.7 to 2.8
• Correct typos and improve phrasing
• Check changelog if there is anything necessary to include into the handbook - nothing necessary
• Correct control volume for cell-centered primary variables in fig. 6.7

Corresponding MR: !3032 (merged)