Clean up redundant and obsolete documentation files

Removed 14 unnecessary .md files:
- CHANGELOG.md (redundant with RELEASE_NOTES)
- COMPARISON.md (content in README)
- CROSS_PLATFORM_FEATURES.md (content in README)
- DELIVERY.md (internal, not for users)
- DESIGN_SHOWCASE.md (obsolete old design)
- DESIGN_SYSTEM.md (obsolete old design)
- GRAFICA_MIGLIORATA.md (Italian, obsolete)
- PROJECT_SUMMARY.md (internal)
- QUICKSTART.md (content in README)
- RIEPILOGO_FINALE.md (Italian, internal)
- RISTRUTTURAZIONE_COMPLETATA.md (Italian, internal)
- UPDATE_INSTRUCTIONS.md (redundant)
- WHATS_NEW.md (replaced by RELEASE_NOTES)
- WINDOWS_EXE_GUIDE.md (redundant with BUILD_WINDOWS)

Kept essential files:
- README.md (main documentation)
- RELEASE_NOTES_v2.0.0.md (release notes)
- RELEASE_CHECKLIST.md (release checklist)
- BUILD_WINDOWS.md (build instructions)
- MACOS_SECURITY.md (macOS security)
- CONTRIBUTING.md (contribution guide)
- docs/OTA_UPDATE_SYSTEM.md (technical docs)

Author: dgiovannetti