Add RDF download button and fix remaining namespace propagation gaps

Adds a third "Download RDF" button (linking to a .ttl file) alongside the
existing YAML/JSON buttons for each of the 3 real-world datasets on the
Working with Data page.

Publishing these surfaced 8 more namespace prefixes (CHEBI, RO, RXNO,
qudt, schema, skos, foaf, adms) with the same root-schema propagation gap
already fixed for hdl/CHMO/ENM: each is declared in a vendored chemdcat-ap
submodule but wasn't visible to SchemaView.namespaces() when the root
schema is loaded directly, so rdflib_dumper fell back to synthesizing
placeholder ns1/ns2/... prefixes bound to the literal (unexpanded) CURIE
string instead of a real URI. Declaring them at the root too produces
clean, fully-resolved .ttl output with zero placeholder prefixes.

scripts/generate_example_outputs.py now also emits a .ttl per real-world
dataset (via the same rdflib_dumper path used by `just test`), regenerated
docs/assets/examples/ and examples/output/ accordingly.

Author: HendrikBorgelt

docs/assets/coremeta4cat_vocabulary.xlsx

@@ -110,6 +110,30 @@ prefixes:
   ENM:
     prefix_prefix: ENM
     prefix_reference: http://purl.enanomapper.org/onto/
+  CHEBI:
+    prefix_prefix: CHEBI
+    prefix_reference: http://purl.obolibrary.org/obo/CHEBI_
+  RO:
+    prefix_prefix: RO
+    prefix_reference: http://purl.obolibrary.org/obo/RO_
+  RXNO:
+    prefix_prefix: RXNO
+    prefix_reference: http://purl.obolibrary.org/obo/RXNO_
+  qudt:
+    prefix_prefix: qudt
+    prefix_reference: http://qudt.org/schema/qudt/
+  schema:
+    prefix_prefix: schema
+    prefix_reference: http://schema.org/
+  skos:
+    prefix_prefix: skos
+    prefix_reference: http://www.w3.org/2004/02/skos/core#
+  foaf:
+    prefix_prefix: foaf
+    prefix_reference: http://xmlns.com/foaf/0.1/
+  adms:
+    prefix_prefix: adms
+    prefix_reference: http://www.w3.org/ns/adms#
   hdl:
     prefix_prefix: hdl
     prefix_reference: https://hdl.handle.net/
@@ -19651,7 +19675,7 @@ classes:
     class_uri: qudt:Quantity
 metamodel_version: 1.7.0
 source_file: coremeta4cat.yaml
-source_file_date: '2026-07-14T17:06:19'
-source_file_size: 7560
-generation_date: '2026-07-14T17:12:30'
+source_file_date: '2026-07-14T17:22:30'
+source_file_size: 7882
+generation_date: '2026-07-14T17:23:49'