ensure docs.vtk.org is buildable
Even though we don't have readthedocs preview yet. We could add a test job that builds the documentation to ensure that it remains build-able (it isn't currently https://readthedocs.org/projects/vtk-docs/builds/22709367/). Building the documentation is fairly quick so I don't expect an impact in CI times. We could make it a weekly job instead of running with every merge request.
cc: @jcfr @ben.boeckel