feat: add RAG converter utilities and update integration guide

RAG Converter:
- Create app/utils/rag_converter.py with conversion functions
- parsed_doc_to_haystack_docs() - convert ParsedDocument to Haystack format
- parsed_chunks_to_haystack_docs() - convert ParsedChunk list to Haystack
- validate_parsed_doc_for_rag() - validate required fields before conversion
- Automatic metadata extraction (dao_id, doc_id, page, block_type)
- Preserve optional fields (bbox, section, reading_order)

Integration Guide:
- Update with ready-to-use converter functions
- Add validation examples
- Complete workflow examples

services/parser-service/app/utils/__init__.py

@@ -0,0 +1,16 @@
+"""
+Utility functions for PARSER Service
+"""
+
+from app.utils.rag_converter import (
+    parsed_doc_to_haystack_docs,
+    parsed_chunks_to_haystack_docs,
+    validate_parsed_doc_for_rag
+)
+
+__all__ = [
+    "parsed_doc_to_haystack_docs",
+    "parsed_chunks_to_haystack_docs",
+    "validate_parsed_doc_for_rag"
+]
+