docs(readme): overhaul README with full TOC and multi-vector-DB integration guide

- Restructure README as TOC-first with 15+ anchored sections
- Add comprehensive 'Using with Different RAG Databases' section covering
  Qdrant (prod), ChromaDB (dev), Pinecone, Weaviate, PGVector, and Milvus
  — each with a complete working adapter and retrieval filter example
- Add 'Drop-in front of your existing RAG stack' guide: explains how to use
  only the /v1/route endpoint to scope retrieval in any existing vector DB
  without migrating data
- Improve Architecture section with 3-scenario ingestion table (A/B/C)
  and annotated directory layout
- Expand Configuration table with Required/Optional column

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: piyushmakhija28