Redesign README and reorganize documentation

README.md:
- Clean, professional layout with tech stack badges
- ASCII architecture diagram for GitHub rendering
- Concise feature highlights in table format
- Clear navigation and quick start guide
- Focus on enterprise patterns and production readiness

Documentation reorganization:
- Rename files for clarity and professionalism
- Add docs/README.md as documentation index
- Update internal cross-references
- Remove redundant/draft files

File renames:
- multidb_rag_chatbot_v3.0.md -> System_Design.md
- Design_v2.0.md -> Architecture_Overview.md
- V1_Summary_and_Roadmap.md -> Roadmap.md
- Full_stack.md -> Enterprise_Architecture.md
- High Level Doc.md -> Service_Layer_Guide.md
- ai_explain.md -> RAG_Tutorial.md
- ai_flow.md -> RAG_Pipeline_Overview.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: asq-sheriff