docs: add good-first-issue walkthrough

[karolswdev]

Goal

Add a short walkthrough for completing a tiny Serfbound documentation PR.

Pointers

• Contributor guide: CONTRIBUTING.md.
• PMO hook docs: pm/roadmap/PMO-CONTRACT.md.
• Docs gate: scripts/check-docs.mjs.

Acceptance

• Shows the path from issue selection to branch, edit, npm run test:docs, .tmp/CONTRACT.md, and PR template.
• Explains when a small docs-only change does not need a roadmap story status flip.
• Links the good first issue label.

No original DOS/Amiga data is needed for this task.