[Docs] Documentation Definition-of-Done for feature work (PR template + checklist)

[pathosDev]

Rationale

Project guideline: a feature is not done until it is properly and thoroughly documented. Nothing enforces that standard today — JSDoc coverage is uneven (#269) and docs sometimes trail features.

Scope

• PR template with a documentation checklist:
  • concept guide on actor-ts.dev (motivation, architecture, HOCON config reference, failure/recovery behavior, operations notes) — English + German in the same change (per AGENTS.md),
  • full JSDoc on new public APIs,
  • runnable example (examples/ or in-guide),
  • CHANGELOG entry (+ migration note if breaking).
• CONTRIBUTING section "Docs are part of the feature" (large modules additionally need their own docs chapter + example app).
• Soft CI reminder: comment when a src/ feature diff ships without a docs/ diff (hint, not a hard fail).

Acceptance

• Template + CONTRIBUTING merged; new feature PRs render the checklist.

Relates

Improvement program M1 (foundation). #26, #269, #300–#306 (i18n).