Remove redundant README_HF.md

Update README and add comprehensive TESTING_GUIDE

Fix: Better error handling, remove doc_id filtering

Fix: Remove global state - make UI stateless per request

Fix: document isolation and session persistence issues

UI refresh: modern gradient design + footer

Fix: Use libgl1 instead of deprecated libgl1-mesa-glx

Add HuggingFace Spaces configuration

Initial commit: VDoc-RAG - Intelligent Document Q&A with RAG