Bugfix/leuven login nodes

[gjbex]

• Reformat to emphasize the node names to use for NX configuration for KU Leuven login nodes
• Replace thinking by genius for KU Leuven Tier-2 NX nodes

[mselwa]

For genius login infrastructure I find the explanation of ssh/NX a little difficult "the nodes listed below can only directly be accessed using SSH, use nx.hpc.kuleuven.be as hostname in the NX client configuraton." (typo in configuration). I would rephrase it:
"the nodes listed below can only directly be accessed in terminal using SSH. To connect with GUI follow the :ref:NX start guide <nx_start_guide.rst>"