docs(config): update documentation for customizing OpenAPI endpoints

Update the existing OpenAPI endpoint documentation and provide detailed
instructions on how to customize the endpoints using configuration options.

Fix incorrect OpenAPI endpoints.

Please see issue #2867

docs/3.config/0.index.md

@@ -40,7 +40,7 @@ Enable experimental features.
 
 #### `openAPI`
 
-Enable `/_nitro/scalar`, `/_nitro/swagger` and `/_nitro/openapi.json` endpoints.
+Enable `/_scalar`, `/_swagger` and `/_openapi.json` endpoints.
 
 - Default: `false`
 
@@ -68,6 +68,22 @@ openAPI: {
 }
 ```
 
+Or if you want to customize the endpoints:
+
+```js
+openAPI: {
+  route: "/_nitro/openapi.json",
+  ui: {
+    scalar: {
+      route: "/_nitro/scalar"
+    },
+    swagger: {
+      route: "/_nitro/swagger"
+    }
+  }
+}
+```
+
 #### `wasm`
 
 Enable WASM support