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/application/big-profile.store.ts

@@ -21,7 +21,7 @@ type Err = Error | undefined;
  * The dashboard data now comes from ONE screen-shaped ("BFF-lite") call that
  * returns registration + person + server-computed `decisions`. One request → one
  * consistent snapshot, instead of stitching three independently loading/erroring
- * resources together client-side. See docs/architecture/0001-bff-lite-decision-dtos.md.
+ * resources together client-side. See docs/reference/architecture/0001-bff-lite-decision-dtos.md.
  */
 @Injectable({ providedIn: 'root' })
 export class BigProfileStore {