docs: add Writing-Scripts page, fix contributing link, add persistent data guidance
- Fix broken CONTRIBUTING.md link (YOUR_USERNAME -> KaiUR) in Home.md
- Add Writing-Scripts.md covering folder structure, header format, naming,
dependencies, and the persistent data pattern
- Update Home.md contributing section to link Writing-Scripts
- Add Contributing section to _Sidebar.md with Writing-Scripts link
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>