docs: reorganize into project/ + reference/, expand Storybook Foundations

Move working docs (backlog, prd, roadmap) under docs/project/ and durable
docs (architecture ADRs, guides, audits) under docs/reference/; add a
docs/README.md index. Update every path reference in code comments, CLAUDE.md,
READMEs, and the new-ssp skill. Expand the Storybook Foundations curriculum
(Overview, BDD, i18n; rename Layers→Domain-Driven Design) and reorder the sidebar.

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

docs/project/backlog/WP-05-parse-boundaries.md

@@ -10,7 +10,7 @@ principle (every response through a hand-written `parse*` returning `Result`).
 
 ## Read first
 
-- `CLAUDE.md` §3 + §4; `docs/architecture/0001-bff-lite-decision-dtos.md`
+- `CLAUDE.md` §3 + §4; `docs/reference/architecture/0001-bff-lite-decision-dtos.md`
 - `src/app/registratie/infrastructure/applications.adapter.ts` (+ its spec — the pattern
   to copy)
 - The three offenders below