Add cover image to v3 announcement post

Editor assessment

A small docs change with a correspondingly small message that does everything right at its scale. `docs` is the correct type, `add cover image to v3 announcement post` is imperative and specific — you know exactly which post and what was added — and the subject is lowercase, period-free, and short. A scope like `blog` might help repo-wide filtering, but for a one-asset addition that's optional. No body is necessary when the diff is a single image reference. Since documentation doesn't ship in the package, the lack of bump implications is exactly what release automation expects from this type. This is the kind of proportionate message that keeps history honest without ceremony.