Provide documentation on when to use the MVC and when to use the WebFlux variant

Is your feature request related to a problem? Please describe.

I think there should be documentation providing reasons for why one would choose the MVC over the WebFlux variant or vice versa.

Describe the solution you'd like

A document with the pros and cons; mentioning developer experience, deployment, operational, and performance implications.

Describe alternatives you've considered

I cannot find any good information other than "some people do not want Netty"/"it's newer".

Additional context

An official performance comparison between the two variants is not strictly part of this PR but it would be nice to have regardless:

https://github.com/spring-cloud/spring-cloud-gateway/issues/3058
https://github.com/spring-cloud/spring-cloud-gateway/issues/2067
https://github.com/spencergibb/spring-cloud-gateway-bench

These do not mention why one would choose one over the other:

Getting Started with Spring Cloud Gateway
Spring Cloud Gateway/Introduction

Start with the Spring Cloud Gateway reference index and the linked Getting Started article to identify where guidance on MVC and WebFlux belongs. Review the linked performance issues and benchmark repository for context, then document the developer experience, deployment, operational, and performance considerations that inform the choice; an official comparison is optional rather than required.