Spring-ai Refine and document initial Modular RAG

• Introduce CompressionQueryTransformer and RewriteQueryTransformer in the Pre-Retrieval Module.
• Extend Query with conversation history and context.
• Remove QueryRouter sub-module. New design will be considered in the next milestone.
• Update RetrievalAugmentationAdvisor accordingly.
• Add initial documentation for Modular RAG, and establish dedicated page for RAG content.

Fixes gh-1850

Comment From: markpollack

great stuff! merged in c7fd7c729e99350c2ff0b54c70db06842d9ce65c