refactor(docs): clean up obsolete documentation

Remove 23 historical/stale documents that are no longer relevant:
- 6 GitHub repository "About section" setup files (one-time action items
  that have already been completed)
- 13 implementation/completion reports from the finished project
  improvement campaign (Phase-1, PART-2.x through PART-5, Context API,
  navigation, and roll-up completion report)
- Project-Layout-and-Structure.md (severely outdated; predates the
  11-module architecture)
- Playground-Implementation-Plan.md (plan fully executed; superseded by
  Playground-Developer-Guide.md)
- Implementation-Status-Report.md (point-in-time snapshot from Oct 2025)
- Project-Improvement-Recommendations.md (all items completed and baked
  into the codebase)

Move docs/Coding Playground Architecture and Implementation.md →
docs/RESEARCH/ alongside the other research reference materials.

Surviving docs/:
- Playground-Developer-Guide.md (active contributor guide)
- Quiz-System-Guide.md (documents the live quiz feature)
- Navigation-Quick-Reference.md (describes current nav behavior)
- RESEARCH/ (all research reference documents)
- SECURITY/ (supply-chain security policy)

Author: mnaimfaizy