Add Open API specification to the documentation (#799)

Author: Kaliumhexacyanoferrat

site/content/docs/test-profiles/_index.md

@@ -10,6 +10,17 @@ Your framework must implement endpoints depending on which test profiles it part
 
 Each profile is run at multiple connection counts to show how frameworks scale under increasing concurrency.
 
+## API Specification
+
+All benchmark endpoints are described in machine-readable OpenAPI 3.0 specifications. Each spec covers request/response schemas, status codes, production and tuned implementation rules, and links back to the relevant documentation pages.
+
+The endpoints are split across two files:
+
+| Spec | Profiles covered | |
+|------|-----------------|---|
+| [openapi.json](/openapi.json) | All HTTP/1.1, HTTP/2, HTTP/3, gRPC, and WebSocket profiles | [Browse](https://redocly.github.io/redoc/?url=https://www.http-arena.com/openapi.json) |
+| [openapi-gateway.json](/openapi-gateway.json) | Gateway profiles (production-stack) | [Browse](https://redocly.github.io/redoc/?url=https://www.http-arena.com/openapi-gateway.json) |
+
 ## Benchmark parameters
 
 Five load generators are dispatched per profile — each one is built for a specific protocol + workload shape. See [Load Generators](../load-generators/) for per-tool details.