[PL/HT] Remove documentation for PorousMedium.

Documentation/ProjectFile/prj/processes/process/HT/porous_medium/i_porous_medium.md

-Tag to specify various porous media.

Documentation/ProjectFile/prj/processes/process/HT/porous_medium/porous_medium/a_id.md

-The 'id' attribute maps the porous medium property to the mesh elements assigned
-to the same 'MaterialIDs' id, where 'MaterialIDs' property vector stored as a
-property withinn the mesh.

Documentation/ProjectFile/prj/processes/process/HT/porous_medium/porous_medium/i_porous_medium.md

-Tag to specify a porous medium property consisting of the permeability, storage
-and porosity constitutive laws.

Documentation/ProjectFile/prj/processes/process/HT/porous_medium/porous_medium/t_permeability.md

-This tag describes the permeability model of the porous medium.

Documentation/ProjectFile/prj/processes/process/HT/porous_medium/porous_medium/t_porosity.md

-Tag describing the porosity model of the porous medium.

Documentation/ProjectFile/prj/processes/process/HT/porous_medium/porous_medium/t_storage.md

-Tag describing the specific storage model of the porous medium.