Add Java and DSL samples to general docs for Service Activator, Transformer, Splitter etc.

[artembilan]

The relevant docs are here:

https://docs.spring.io/spring-integration/docs/6.1.0-M2/reference/html/messaging-endpoints.html#service-activator
https://docs.spring.io/spring-integration/docs/6.0.4/reference/html/message-transformation.html#transformer
https://docs.spring.io/spring-integration/docs/6.0.4/reference/html/message-routing.html#filter
https://docs.spring.io/spring-integration/docs/6.0.4/reference/html/message-routing.html#router-namespace
https://docs.spring.io/spring-integration/docs/6.0.4/reference/html/message-routing.html#splitter

The link to annotation configuration and Java (and other DSLs) are good addition, too.