docs(form-builder): document S3a PR 2 complex field types and update FORM-05 stub note

- Add VitePress pages for AVAILABILITY_PICKER and SECTION_PRIORITY
  and a TAG_PICKER configuration note. Wire them into the organisator
  sidebar under a new Formulieren section alongside the existing
  "Wat is een formulier" page.
- BACKLOG.md: nuance FORM-05 — the stub-shaped behaviour for public
  event_registration submissions is already shipping via the existing
  TriggerPersonIdentityMatchOnFormSubmit listener (writes 'pending').
  The real work (PersonIdentityService::detectMatchesByValues + an
  extra branch in resolveStatus) is what remains. Added a done entry
  for S3a PR 2 to the Opgeloste items list.
- API.md: add VALIDATION_FAILED to the public-form error code table
  and document the SECTION_PRIORITY shape error messages (Dutch copy
  served under errors."values.{slug}").
- COPY_CATALOGUE.md: new S3a PR 2 section capturing the seeder
  help_text, the IdentityMatchBanner copy (clearly marking the
  backend message as authoritative), all empty/error state copy for
  the three new components, and the SECTION_PRIORITY shape error
  strings.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dev-docs/API.md

@@ -809,10 +809,29 @@ Codes:
 | `SCHEMA_NOT_FOUND`            | 404  | public token does not resolve                  |
 | `SUBMISSION_ALREADY_SUBMITTED`| 409  | submit on finalised submission                 |
 | `RATE_LIMITED`                | 429  | includes `Retry-After` header                  |
+| `VALIDATION_FAILED`           | 422  | per-field validation — see `errors` map below  |
 
 Authoritative source: `api/app/Exceptions/FormBuilder/PublicFormApiException.php`
 and its concrete subclasses.
 
+`VALIDATION_FAILED` returns the `errors` field keyed by `values.{slug}`
+with an array of Dutch messages. Field-shape triggers include:
+
+- **SECTION_PRIORITY** — values must be `{ section_id, priority }[]`
+  with unique section_ids and priorities in `1..5`, max 5 entries, and
+  section_ids scoped to the schema's owner event tree. Specific
+  messages land under `errors."values.{slug}"`:
+  - `"Dezelfde sectie mag slechts één keer worden opgegeven."`
+  - `"Elke prioriteit mag slechts één keer worden toegekend."`
+  - `"priority moet tussen 1 en 5 liggen (positie {n})."`
+  - `"Je kunt maximaal 5 voorkeuren opgeven."`
+  - `"Eén of meer secties horen niet bij dit evenement."`
+  - `"Ongeldig formaat voor sectievoorkeuren."` / `"Ongeldig voorkeur-element op positie {n}."`
+    / `"section_id ontbreekt op positie {n}."` / `"priority ontbreekt of is ongeldig op positie {n}."`
+
+Authoritative source for shape rules:
+`api/app/Services/FormBuilder/FormValueService::validateSectionPriorityShape`.
+
 ### `GET /public/forms/{public_token}`
 
 Returns `PublicFormSchemaResource`. Shape: