/rdf/ endpoint to retrieve our data as LOD

Collections, and possibly other RDF entities, currently get IRIs under the `/rdf/*` path prefix. The Django backend does not recognize those paths (at least not in the case of `/rdf/collections/*`), so the user is redirected to the index page.

Possible fixes:

| Fix | Pro | Con |
|---|---|---|
| Rename `/rdf/*` entities to `/api/*` | Straightforward | Departs from #156 |
| Move RDF-producing endpoints from `/api/*` to `/rdf/*` | Arguably correct | Requires splitting the API router |
| Change all `/api/` routes to `/rdf/` | Straightforward | Only valid if all API endpoints produce RDF data |

Start by reading the Django backend route definitions for `/rdf/collections/*` and the current `/api/` endpoints, then review the linked discussion in #156. Determine which routing approach preserves the intended LOD IRIs and identify the affected RDF entities. Done means RDF entity URLs resolve to their data rather than redirecting to the index page.

Written by the indexing model from the issue text.