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

What this MR does / why does DuMux need it: This solves #1385 (closed)

Notes for the reviewer

TODO: insert text here

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)
• 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.
• if the examples are modified, is the documentation regenerated (using generate_example_docs.py)

Closes #1385 (closed)