docs(portal-self-service): record NL DS + DigiD decisions and demo note (refs #67)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/demo-script.md

@@ -5,6 +5,29 @@ copy-pasteable walkthrough against a local `make up` stack.
 
 ---
 
+## S-08c — Self-service submit form (NL Design System + DigiD)
+
+**Outcome:** a zorgprofessional logs in via mock DigiD and submits a BIG registration through the
+self-service portal (NL Design System styling); the page confirms with the reference returned by the
+BFF. The bsn comes from the DigiD token, so it's a confirm-and-submit flow (no bsn field).
+
+```bash
+# 1. Bring the backend + Keycloak up (BFF on :8080, Keycloak on :8180).
+make up
+
+# 2. Serve the portal (dev server); it redirects to Keycloak for DigiD login.
+pnpm nx serve self-service        # → http://localhost:4200
+
+# 3. In the browser: log in as the mock DigiD user jan-burger / test123, then submit.
+#    The page shows the returned registration reference.
+```
+
+> First real UI. The full **login → submit → success** happy path is automated in **S-08d**
+> (Playwright, against the compose-served app). Component tests + an axe WCAG 2.1 AA check on the
+> submit page run headless in the `frontend` CI lane. See `docs/frontend-decisions.md`.
+
+---
+
 ## S-08a — Nx workspace + self-service portal skeleton
 
 **Outcome:** the frontend foundation — an Nx (pnpm) monorepo with the `self-service` Angular app