[IMPROVEMENT] Add documentation for other open-source CGMES converter

[petersalemink95]

Power Grid Model IO currently does not have a CGMES converter implemented. However, there is an open-source CGMES converter available through https://github.com/SOPTIM/cgmes2pgm_converter.

Let's add documentation about that converter to our converter documentation. We might also add an example for it.

[mgovers]

For reference: there is a separate ticket for the actual incorporation in #207

[scud-soptim]

Thank you, I would like to add documentation for https://github.com/SOPTIM/cgmes2pgm_converter