chore: add WBG OSS compliance files and improve README

[avsolatorio]

This PR adds all files required for the World Bank Group Open Source Code Catalog
compliance checklist, updates the project license from Apache 2.0 (Vercel origin)
to MIT + WB IGO Rider, and significantly improves the README for public audiences.

Changes

License (Requirement 3)

• Replaced LICENSE — Apache 2.0 (Vercel, Inc.) → MIT License (The World Bank)
• Added WB-IGO-RIDER.md — World Bank IGO procedural rider covering
  privileges/immunities, dispute resolution, and governing law
• Added NOTICE — preserves required attribution to the original Vercel AI Chatbot
  (Apache 2.0)

New compliance files

• CITATION.cff — CFF citation metadata for the AI for Data team (Req 6)
• CODE_OF_CONDUCT.md — official World Bank / Contributor Covenant template (Req 7)
• CONTRIBUTING.md — official World Bank contributing guide template (Req 8)
• THIRD_PARTY_LICENSES.md — full third-party license inventory organized by
  frontend (Node/pnpm) and backend (Python/uv), generated from pnpm licenses list
  and pip-licenses (Req 9 / WBG Section 7)

README overhaul (Requirement 1)

• Rewrote description to clearly explain the MCP + PCN integration and target audience
• Added Architecture section with system diagram and tech stack tables for both
  frontend and backend
• Added Configuration section with full env variable reference tables for backend
  and frontend
• Added Authentication section covering all four auth modes (guest, user,
  msal, data360) with MSAL setup instructions
• Added Deployment section covering Docker dev/prod, Azure App Service, and Vercel
• Added Development section with test commands, code style table, and migration
  workflow
• Added Documentation table linking to all sections of the hosted site
• Added Contact, Contributing, and Citation sections
• Added required WBG license notice (exact required text)

WBG checklist status after this PR

#	Requirement	Status
1	README.md	✅
2	Repository Details (GitHub settings)	Manual — set description, website, topics
3	License (MIT + WB IGO Rider)	✅
5	Documentation	✅
6	CITATION.cff	✅
7	Code of Conduct	✅
8	Contributing Guide	✅
9	No secrets / no data files	✅
10	Bibliography	N/A

Follow-up actions before submission

• Set GitHub repository About description, website URL, and topics