docs: add "Built on atjam" showcase page
New /docs/built-on (TSX route + markdown mirror): a truthful "in the wild"
adopters table (the atjam.at app itself + EPTSS) plus an end-to-end worked
example mapping a judged, votable game jam — the rpg.actor shape — onto the
at.atjam.* spine. It shows the two illustrative actor.rpg.* lexicons an
adopter would publish under their own namespace, the round wired through the
extension points, and vote tallying via the existing Queries.getBacklinks
substrate. Explicit callout that rpg.actor does not actually run on atjam.
Wired into the docs sidebar, the overview list, and the footer.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>