Add gateway api ref

Signed-off-by: bitliu <[email protected]>

Author: Xunzhuo

docs/index.rst

@@ -15,6 +15,9 @@ standalone or Kubernetes-based application gateway.
    This project is under active development. Many, many features are not
    complete. We would love for you to :doc:`get involved<get_involved>`.
 
+   This project is now listed in official 
+   `Gateway API implementations <https://gateway-api.sigs.k8s.io/implementations/#envoy-gateway>`_ .
+
 .. toctree::
   :maxdepth: 1