JIRA: https://jira.spring.io/browse/INT-4277 *Mostly migration from the https://github.com/spring-projects/spring-integration-java-dsl/wiki/Spring-Integration-Java-DSL-Reference * Add cross links from some appropriate chapters * Fix log message in the `WebSocketInboundChannelAdapter` * Document `IntegrationFlowContext` * Document gateways via `IntegrationFlow`