Enhance format validation for ontology files

[coderabbitai]

The current implementation accepts non-RDF/XML formats with a warning, but the requirement states that ontologies must be in RDF/XML format.

Consider implementing stricter format validation to ensure compliance with this requirement.

Requested by @anitacaron in PR #141 comment.

[matentzn]

I am pretty sure I addressed that by using "robot metrics" to determine the serialisation format? Maybe this does not work anymore..

[anitacaron]

Then I need to update the documentation.

[matentzn]

Best ensure I am not making this up.. I hallucinate more than any LLM I know of recently