chore: make gateway api reference visible

Signed-off-by: Sergei Lukianov <[email protected]>

Author: Frostman

docs/reference/gateway-api.md

@@ -0,0 +1,4 @@
+It's an automatically generated reference for the Gateway API.
+You can find latest version in the [Gateway repository](https://github.com/githedgehog/gateway/blob/master/docs/api.md).
+
+--8<-- "docs/reference/gateway-api.md.gen"