Naming conventions when resolving xref keyword

[mkuehbach]

Relevant for semantic mapping after Sept NIAC meeting:
For the conversion direction yaml2nxdl xref is currently resolved as such:
f"This concept is related to term {xref_entries.get('term', 'NO TERM')}_ "
f"of the {xref_entries.get('spec', 'NO TERM')} standard.\n\n"

But that xref is not necessarily always a "standard" but sometimes an "ontology",
therefore suggesting to revise this at some point.

[lukaspie]

We could also think about having another keyword for xref that describes what kind of entity we are referencing, i.e., ontology, standard, defined vocabulary, etc.