spring-cloud-gateway: Document that spring-cloud-gateway-mvc is not compatible with spring-cloud-gateway-core

This is similar to #319 . Application created using http://start.spring.io with gateway dependency and with additional spring-cloud-gateway-mvc fails.

 <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-gateway-mvc</artifactId>
  </dependency>

If spring-boot-starter-web is not compatible, then what is the purpose of this dependency?

Error:

Consider defining a bean of type 'org.springframework.http.codec.ServerCodecConfigurer' in your configuration.

Documentation: https://cloud.spring.io/spring-cloud-gateway/multi/multi__building_a_simple_gateway_using_spring_mvc_or_webflux.html