Currently we only have examples of `EXACT` and `PATHPREFIX` path matching in the advanced routing [guide](https://docs.nginx.com/nginx-gateway-fabric/traffic-management/advanced-routing/). Once this [PR](https://github.com/nginx/nginx-gateway-fabric/pull/3874) goes gets merged to main, we should update the guide to showcase an example of `RegularExpression` path matching. Acceptance Criteria: - Update the advanced routing [guide](https://docs.nginx.com/nginx-gateway-fabric/traffic-management/advanced-routing/) with valid examples for `RegularExpression` path matching - Update the gateway API compatibility doc - We need to update the official documentation and also update the example in our [repo](https://github.com/nginx/nginx-gateway-fabric/tree/main/examples/advanced-routing)
