WIP: [doxygen] Add doc for grid geometry.

Closed Ivan Buntic requested to merge doc/gridgeometry into master

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:

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