docs: align README and repo structure with agent-framework (#271)

- Restructure README to match microsoft/agent-framework layout:
  Banner → Getting Started → Installation → Documentation → Highlights →
  Quickstart → Examples → Packages → OWASP → Performance → Contributors
- Add SVG banner image (docs/assets/readme-banner.svg)
- Add COMMUNITY.md (community resources, related projects)
- Move Architecture Notes to docs/ARCHITECTURE.md (trim README)
- Organize 16 proposal docs into docs/proposals/ subdirectory
- Add 'We want your feedback' and 'Important Notes' sections
- Fix broken PROPOSALS-INDEX.md link in RELEASE_NOTES_v1.0.0.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: imran-siddique