docs: declare where the data comes from, and under what terms

The module's own Apache-2.0 licence covers its code and says nothing
about the material it fetches. STORE.yaml now carries a data_source
block (publisher, url, terms, required attribution, and bundled:
false), and MODULE.md/MODULE.de.md explain it in both languages,
including the sentence that the retrieved text does not replace the
official version.

Enforced platform-side by data_source_gap in chain_hub::store_index;
the four rules are written up in docs/architecture/store-format.md of
the platform repo.

Signed-off-by: flemming-it <sf@flemming.it>

STORE.yaml

@@ -7,6 +7,17 @@ author:
 
 license: Apache-2.0
 
+# Where the data comes from, and under what terms. The `license`
+# above covers this module's code; this block covers the material
+# the module reaches. See docs/architecture/store-format.md.
+data_source:
+  name: "EUR-Lex / CELLAR (Publications Office of the EU)"
+  url: https://eur-lex.europa.eu/
+  license: "Wiederverwendung nach Beschluss 2011/833/EU; redaktionelle Inhalte CC BY 4.0"
+  attribution: "Quelle: EUR-Lex, © Europäische Union, 1998-2026"
+  # Never true: the module fetches at runtime and ships no corpus.
+  bundled: false
+
 tagline:
   en: "Fetch EU legal acts from EUR-Lex by CELEX"
   de: "EU-Rechtsakte per CELEX von EUR-Lex holen"