SpringBoot Finalize and document Spring Boot's HTTP Service client support

Now that @HttpServiceClient support has been dropped from Spring Framework 7.0 and removed Spring Boot in #47123 we need consider how we should proceed for Spring Boot 4.0.

At a very minimum, we at least need to add some documentation.

As it stands, we no longer to scan for HTTP Service clients, however, we do still have configuration properties and HttpServiceGroupConfigurer support. We've decided to proceed with this approach for 4.0 and gather more feedback about if something similar to @HttpServiceClient would be useful in the future.

The primary use-case for @HttpServiceClient of having multiple client interfaces in multiple groups. i.e.:

@HttpServiceClient("customer")
public interface CustomerService {
}

@HttpServiceClient("sales")
public interface SalesService {
}

@HttpServiceClient("address")
public interface AddressService {
}

can still be covered with the @ImportHttpServices annotation:

@SpringBootApplication
@ImportHttpServices(group = "customer", types = CustomerService.class)
@ImportHttpServices(group = "sales", types = SalesService.class)
@ImportHttpServices(group = "address", types = AddressService.class)
public class MyApplication {
}