Add complex-document-rag under generative-ai-service

[brnil6]

This PR adds the Complex Document RAG project to ai/generative-ai-service/complex-document-rag.

Includes:

• README with copyright and usage information
• files/ directory containing all source code, handlers, agents, and supporting scripts
• sample data and queries for demonstration and testing
• .gitignore to exclude local environment and cache files

Purpose:
Complex Document RAG for integration into the Generative AI Service examples repository.

All unnecessary artifacts (venv, pycache, .DS_Store, logs) have been excluded.