docs: fix HTTP discovery endpoint response to match Rust implementation (#2883)
Update the GET / example response in docs/http.md to reflect the actual
fields returned by the coglet Rust server (routes.rs):
- Fix docs_url value: /openapi.json -> /docs
- Fix key name: health_check_url -> healthcheck_url
- Add missing fields: shutdown_url, predictions_idempotent_url,
predictions_cancel_url
- Document all three training fields instead of just trainings_url
Closes #2873