[doc][examples] Add paraview description for porenetwork representation

Feature request Add a description of how to generate the pore bodies in paraview in the example of the porenetwork model.

What does this feature / why does DuMux need it: It would be nice to have a short description within the README.md on how to get the same representation of the porenetwork in paraview as shown in the example image. New users - and also those without much paraview experience - would benefit from that.

Which issue does this feature fix (if any)

Anything else we need to know?: