Evaluate github.io content-type behavior

[JohannesLipp]

As described in #302 , the response content-type for https://international-data-spaces-association.github.io/InformationModel/docs/serializations/ontology.json is application/json, although application/ld+json is requested. Please verify how that content-type is generated, and if it is possible to alternate that. For example, via a mini experiment with custom file extensions like foo.json, foo.jsonld, foo.bar.