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-17-app-a11y.md

@@ -41,7 +41,7 @@ Three app-level gaps close the WCAG story:
 - `src/app/app.config.ts` + a new small `shared/` focus-on-navigation service
 - `eslint.config.mjs`, `package.json` (angular-eslint)
 - Any template the new lint rules flag
-- New `docs/wcag-checklist.md`
+- New `docs/reference/wcag-checklist.md`
 - New `src/docs/a11y.mdx` — title `Foundations/Accessibility`
 
 ## Steps
@@ -50,7 +50,7 @@ Three app-level gaps close the WCAG story:
    focus lands on the new page's heading).
 2. Add angular-eslint; enable the template rules on inline templates; plant a bad
    pattern, see it fail, remove it; fix real hits.
-3. `docs/wcag-checklist.md`: manual checks per page (dashboard, wizards, brief, login) —
+3. `docs/reference/wcag-checklist.md`: manual checks per page (dashboard, wizards, brief, login) —
    keyboard walk & focus order, no traps, 200% zoom/reflow, NVDA or VoiceOver pass,
    visible focus, error announcement; status columns (page × check).
 4. `src/docs/a11y.mdx`: the layered approach — axe gate (WP-01) + template lint + play
@@ -75,7 +75,7 @@ Three app-level gaps close the WCAG story:
   setup. Less code to hand-maintain, same coverage plus more.
 - The dashboard's checklist pass surfaced a **real bug**: `aanvraag-block`'s warning
   `app-alert` (two `app-button` actions) overflows the viewport at 320px — its
-  `.feedback` flex row doesn't wrap. Documented in `docs/wcag-checklist.md` with the
+  `.feedback` flex row doesn't wrap. Documented in `docs/reference/wcag-checklist.md` with the
   root cause, **not fixed** — fixing live component CSS found via the checklist is the
   "full manual audit" scope this WP's Out-of-scope section explicitly defers, not this
   WP's own deliverable. Flagged here so it isn't lost.