docs(architecture): ADR-0004 — Reqnroll as BDD framework (refs #5)

Record the choice of Reqnroll.xUnit for the acceptance layer introduced in
S-04, and register the previously-unlisted ADR-0002/0003 in the MkDocs nav.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mkdocs.yml

@@ -23,6 +23,9 @@ nav:
   - Product Requirements: PRD.md
   - Architecture:
       - "ADR-0001: Loose coupling": architecture/adr-0001-loose-coupling.md
+      - "ADR-0002: Catalogus design": architecture/adr-0002-catalogus-design.md
+      - "ADR-0003: ACL default-fill": architecture/adr-0003-default-fill.md
+      - "ADR-0004: BDD framework": architecture/adr-0004-bdd-framework.md
   - Working in Gitea: gitea-workflow.md
   - Runbooks:
       - CI: runbooks/ci.md