handle traffic to old releases page in docs

[danabauer]

Back in the day our release information lived in a Docusaurus blog at docs.overturemaps.org/releases. The release calendar and history now lives at https://docs.overturemaps.org/release-calendar/. Some folks are still hitting the old URL and getting 404s. We could create a redirect but we might also be able to handle this with a slug in the top matter of the markdown page.