feat: enhance LLMClient with structured output capabilities

feat: enhance architecture and developer documentation for clarity and detail

Revise documentation in app.py for Deep‑Research PDF Field Extractor, enhancing clarity on system architecture, core components, processing pipeline, and key features. Update usage instructions and support resources for better user guidance.

Update Dockerfile to use new app entry point and enhance requirements.txt with additional dependencies. Remove obsolete streamlit_app.py file.