fix(ci): docs:check-links groen — exclude docs/old/ + archiveer stale plans

CI faalde sinds #191 (docs cleanup) op pre-existing broken links:
- docs/old/ bevat archief-docs met by-design stale paden
- docs/plans/PBI-79*, M9*, M11* hadden geprojecteerde paden naar
  ../backlog/index.md (verplaatst naar docs/old/backlog/) en naar
  app-bestanden die nooit met de juiste relatieve prefix waren geschreven
- docs/adr/0000* verwees naar docs-restructure-ai-lookup.md (verplaatst)
- docs/glossary.md verwees naar /docs/backlog/index.md (verplaatst)

Fixes:
- scripts/check-doc-links.mjs: skip docs/old/ recursief
- Move docs/plans/{PBI-79,M9,M11}*.md → docs/old/plans/ (allemaal merged PBIs;
  plans waren historisch)
- docs/adr/0000-record-architecture-decisions.md: update pad naar archief
- docs/glossary.md: verwijder dode "backlog index"-link

Verificatie: `npm run docs:check-links` → ✓ All doc links valid (105 files)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/adr/0000-record-architecture-decisions.md

@@ -61,6 +61,6 @@ that supersedes the old one rather than editing the original.
   not enforced through review.
 - Backfilling existing decisions requires writing 5–8 retrospective ADRs
   for choices that were never recorded (planned in fase 6 of
-  [`../plans/docs-restructure-ai-lookup.md`](../plans/docs-restructure-ai-lookup.md)).
+  [`../old/plans/docs-restructure-ai-lookup.md`](../old/plans/docs-restructure-ai-lookup.md)).
 - Two templates means a per-decision choice about which to use. Mitigated
   by making Nygard the explicit default in `README.md`.