Spring-ai Add ETL diagram to documentation

Comment From: tzolov

@markpollack Added generic ETL diagram https://docs.spring.io/spring-ai/reference/0.8-SNAPSHOT/api/etl-pipeline.html#_api_overview And detailed one at the bottom: https://docs.spring.io/spring-ai/reference/0.8-SNAPSHOT/api/etl-pipeline.html#etl-class-diagram

Comment From: tzolov

Resolved