Fix a document 404ing with a redirect

www.agileplannerapp.com/boards/173808/cards/5678

it seems 2 editors started working on separate drafts for the same content, hence there were duplicate documents. the content lives at the url that contains the slug with a sequence number. hence redirecting to that location.

PR for router-data: https://github.gds/gds/router-data/pull/145