Add guidance on usage of additional themes for datasets

[jakubklimek]

Based on https://github.com/SEMICeu/DCAT-AP/issues/314, https://github.com/SEMICeu/DCAT-AP/issues/207, and the description of dcat:theme, the chapter on Other controlled vocabularies, I think there is a need for clarification of usage of additional dataset themes, including examples. It is clear that Data theme vocabulary needs to be used for dcat:theme. What is unclear from the current state of DCAT-AP 3.0.0 and where the discussions are not yet concluded is how additional themes should be used. Let's say I want to use Eurovoc in addition to Dataset theme. What do I do?

The usage note for dcat:theme says: "The values to be used for this property are the URIs of the concepts in the vocabulary." It is unclear whether it is ONLY values from the vocabulary, or AT LEAST ONE value from this vocabulary.

Option 1 (implemented in the Czech National Open Data Catalog): Use dcat:theme also for Eurovoc, e.g.:

<dataset1> dcat:theme <http://publications.europa.eu/resource/authority/data-theme/TRAN> .
<dataset1> dcat:theme <http://eurovoc.europa.eu/1001> .

This seems to be discouraged by @bertvannuffelen in https://github.com/SEMICeu/DCAT-AP/issues/207#issuecomment-1700613026 and creation of subproperties of dcat:theme and enforcing the ONLY values from the dataset theme vocabulary policy is suggested. However, as I mentioned in https://github.com/SEMICeu/DCAT-AP/issues/314#issuecomment-1765711454, I do not think that these two approaches go together, as from the RDF point of view, the values of a subproperty can be interpreted also as values of the superproperty, i.e. dcat:theme, violating the constraint.

Option 2 Other vocabularies use dct:subject:

<dataset1> dcat:theme <http://publications.europa.eu/resource/authority/data-theme/TRAN> .
<dataset1> dct:subject <http://eurovoc.europa.eu/1001> .

This is another approach suggested by @bertvannuffelen in https://github.com/SEMICeu/DCAT-AP/issues/314#issuecomment-1764792636, which does not create any problems. However, it is not mentioned anywhere in DCAT-AP.

I think this shows the need for a decision and a clearer guidance on how additional dataset themes should be used in DCAT-AP.