WIP: [doxygen] Add doc for grid geometry.

What this MR does / why does DuMux need it:

This MR adds a first try for a documentation on the gridgeometry.

Notes for the reviewer

Follow the instructions on https://dumux.org/docs/doxygen/master/building-the-documentation.html to build the documentation locally. The added description can be found under "Module documentation -> Discretization schemes".

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

• does the new code follow the style guide?
• do the test pipelines pass? (see guide on how to run pipelines for a merge request)