add `soil_surface_water_unsaturated_zone__exfiltration_volume_flux` to the docs

[atsiokanos]

Wflow version checks

• I have checked that the issue still exists on the latest versions of the docs on master here

Kind of issue

Docs are missing

Location of the documentation

https://deltares.github.io/Wflow.jl/dev/model_docs/parameters_landhydrology_sbm.html

Documentation problem

soil_surface_water_unsaturated_zone__exfiltration_volume_flux is missing from the parameters/variable table. Add it in a similar way to soil_surface_water_saturated_zone__exfiltration_volume_flux

Suggested fix for documentation

No response

[vers-w]

As part of this PR #730 work this variable is not needed anymore.