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>

src/app/registratie/contracts/brp-address.dto.ts

@@ -3,7 +3,7 @@
  *
  * In production this is GENERATED from the OpenAPI/TypeSpec spec and served by our
  * own backend, which talks to the BRP behind an adapter. The frontend never sees
- * the BRP's own wire format. See docs/architecture/0001-bff-lite-decision-dtos.md.
+ * the BRP's own wire format. See docs/reference/architecture/0001-bff-lite-decision-dtos.md.
  *
  * "Geen adres bekend" is a first-class outcome (`gevonden: false`), not an error —
  * the wizard falls back to manual entry (PRD §7). Slice 1 ships only the happy

src/app/registratie/contracts/dashboard-view.dto.ts

@@ -10,7 +10,7 @@
  * In production these types are GENERATED from the OpenAPI/TypeSpec spec (one
  * source of truth for both sides), and the `decisions` block is computed BY THE
  * BACKEND — never recomputed on the client. The frontend renders decisions; it
- * does not own the rules. See docs/architecture/0001-bff-lite-decision-dtos.md.
+ * does not own the rules. See docs/reference/architecture/0001-bff-lite-decision-dtos.md.
  *
  * One screen-shaped call replaces the previous three (BIG-register + BRP + …),
  * so the page always sees one consistent snapshot instead of three independently