[handbook] Update to 3.7, update discretization images and some text.

What this MR does / why does DuMux need it:

This MR updates the versions of Dumux and Dune in the handbook. Also, the images for tpfa, mpfa and box have been unified and the text for the box method has been updated.

Notes for the reviewer

You need to compile the pdf of the handbook locally.

Before you request a review from someone, make sure to revise the following points:

• do the test pipelines pass? (see guide on how to run pipelines for a merge request)
• all files have to end with a \n character. Make sure there is no \ No newline at end of file comment in "Changes" of this MR.