- Add Refine stage between Parse and Chunk stages
- Enable RefiningOptions by default in ChunkingOptions
- Set ConvertToMarkdown=true by default for RAG optimization
- Add IMarkdownConverter and IImageToTextService to DocumentProcessor
- Add RefiningOptions.ForRAG and ForRAGWithImages factory methods
- Update ARCHITECTURE.md with 5-stage pipeline documentation

Pipeline: Extract → Parse → Refine → Chunk → Enhance