Enable automated schema-repo trigger for docs publication (schema repo)

[lowlydba]

Sub-issue of #679, scoped to the OvertureMaps/schema half of the work.

Add a workflow that fires when overture-schema cuts a new release and dispatches a production rebuild of OvertureMaps/docs against that release, so the docs site's generated schema reference stays in sync automatically. Cross-repo dispatch needs the overture-releaser app (#637) extended with Actions: write and installed on OvertureMaps/docs, tracked separately in #689.

Implemented in #690.