docs: add static API overview page

Author: jung-thomas

site/api/index.md

@@ -0,0 +1,24 @@
+---
+title: API Reference
+---
+
+# API Reference
+
+The Star Wars CAP Showcase exposes its services on three protocols simultaneously:
+
+| Protocol | Base path | Best for |
+| --- | --- | --- |
+| OData v4 | `/odata/v4/<Service>/` | Fiori UI, SAP integration, standard tooling |
+| REST | `/rest/<Service>/` | Simple HTTP clients, scripting |
+| GraphQL | `/graphql/` | Flexible ad-hoc queries, developer tooling |
+
+## Services
+
+| Service | Description | Full API reference |
+| --- | --- | --- |
+| **DataService** | Read-only projection of all Star Wars entities — Films, People, Planets, Species, Starships, Vehicles, Shows, Episodes | [Data Service API](/api/data-service) |
+| **StarWarsFilm** | Draft-enabled Film CRUD with junction navigation | [Film API](/api/film) |
+
+The generated reference pages below are produced by [Widdershins](https://github.com/Mermade/widdershins) from the OpenAPI 3.0 specs at `cap/docs/*.openapi3.json`.
+
+> **Tip:** Start the server with `npm run sqlite` and browse to `http://localhost:4004/api-docs` for an interactive Swagger UI that covers all services.