Add/Move Validate Custom Namelist to ICON docs

[AnnikaLau]

We should add the content of Validate Custom Namelist to the official ICON docs in a more general form.

We should also add an explanation why mpi and nproma tests are failing on Alps. It could also be possible to do a tolerance test for that.

[AnnikaLau]

Working on a MR Documentation for testing customized namelists