Add Java config examples to XML and XMPP docs

Modernize documentation by adding Java-based configuration examples
alongside existing XML namespace examples for XML transformers,
XPath splitters, and XMPP adapters.

The documentation previously only showed XML namespace configuration.
As Spring has moved toward Java configuration with `@Configuration`
classes and `@Bean` methods, developers need examples showing the
modern approach using `IntegrationFlow` and direct bean configuration.

This change adds Java examples for:
- `UnmarshallingTransformer` and `MarshallingTransformer` with
  `Jaxb2Marshaller`
- `XsltPayloadTransformer` with resources, templates, and XSLT
  parameters
- `ResultTransformer` implementations with various result types
- `XPathMessageSplitter` with polling and document creation
- `XmppConnectionFactoryBean` for XMPP connections
- `ChatMessageListeningEndpoint` and
  `ChatMessageSendingMessageHandler` for XMPP messaging
- `PresenceListeningEndpoint` and `PresenceSendingMessageHandler`
  for presence handling

Author: cppwfs