[GeoMechanicsApplication] Add test and documentation for partially saturated flow for Pw and U-Pw elements on quadrilateral elements

[indigocoral]

The partially saturated flow, specifically with the retention law of "saturated_below_phreatic_level" is not giving correct results. Flux goes toward the top of the column above the phreatic line.

• Add test case(s)
• Documentation for test cases

The investigation issue and the fix to the problem can be found here: INVESTIGATION - Partially saturated flow does not work as expected

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UPDATE 11/12/2024 Refinement
The test cases will be added but will be skipped as a reminder that this issue needs to be fixed sometime.

[matekelemen]

Unrelated comment here: I just noticed that we have a Task tag under "Type" (you can set it on GitHub on the right hand side bar). @KratosMultiphysics/geomechanics can you please tag tasks as such to help navigation a bit? (no need to do it retroactively)