Docs/diagram: make DCAT reuse explicit; avoid implying new subclasses

[jgeluk]

Tracks part of #98. Clarify diagrams/docs to show reuse of dcat:Dataset, dcat:Distribution, and dcat:DataService. If subclass arrows are shown, make them dashed/no arrowheads and add explicit namespaces. Refs #98

[jgeluk]

Clarification: make DCAT reuse explicit; avoid implying new subclasses

Issue: #138 tracks updating diagrams/docs so they clearly show reuse of DCAT classes (not new subclasses), per discussion in #98.

What to change (exact repo artifacts)

• Diagrams:
  • Edit assets/dprod-model.drawio (source).
  • Re-export assets/dprod-model.svg (from the .drawio).
  • After rebuild, these are copied to dist/assets/ by the build.
• Docs text:
  • Update ontology/dprod/README.md to explicitly state that dcat:Dataset, dcat:Distribution, and dcat:DataService are reused (not redefined), and only dprod:DataProduct is introduced (as a subclass of dcat:Resource).
  • Verify dist/index.html (generated site) reflects the updated wording and shows the updated diagram.

How the diagram should look

• Prefix all reused DCAT classes directly in the boxes: e.g., dcat:Dataset, dcat:Distribution, dcat:DataService.
• Prefix DPROD classes/properties with dprod: (e.g., dprod:DataProduct).
• Show dprod:DataProduct rdfs:subClassOf dcat:Resource as a proper subclass arrow (since it is).
• For relationships indicating reuse/association (not subclassing), use dashed lines without arrowheads or a labeled association (e.g., "uses DCAT class") instead of subclass arrows.
• Add a small legend clarifying: solid arrow = subclass; dashed line = reuse/association.

Optional diagram annotations

Box labels:
- dprod:DataProduct (subClassOf dcat:Resource)
- dcat:Dataset
- dcat:Distribution
- dcat:DataService

Relationships (examples):
- dprod:DataProduct ——— (dashed) ——— dcat:Dataset
- dprod:DataProduct ——— (dashed) ——— dcat:DataService

Acceptance criteria

• The updated .drawio and regenerated .svg are committed under assets/.
• ontology/dprod/README.md contains an explicit statement that DCAT classes are reused; only dprod:DataProduct is newly defined (subClassOf dcat:Resource).
• The published site (dist/index.html + dist/assets/*) after rebuild shows the new diagram and wording.
• The diagram contains clear prefixes and no subclass arrows for dcat:Dataset, dcat:Distribution, dcat:DataService.

Refs #98