docs: add Community Maps player-guide section

[karolswdev]

Goal

Add a short player-facing Community Maps section to docs/player-guide.md.

Pointers

• UI behavior: packages/app/src/main.ts around the Community Maps panel and Playing community map state.
• Local storage: packages/app/src/community-map-library-store.ts.
• Delivery evidence: pm/roadmap/serfbound/phase-43-community-maps/evidence-story-07.md.

Acceptance

• Explains browse, download/library, play, and the import/licensed-package requirement in player language.
• Mentions that custom maps are user-authored map data, not original game assets.
• Runs npm run test:docs.

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