Manually review and update the City Pairs OAS spec

This one was just auto-converted from 2.0.

Consider going through a tutorial to validate all the sections and see how they display in the doco.

The issue concerns the City Pairs OAS specification and how its sections display in the documentation. Start by locating the converted specification and reviewing an applicable OpenAPI tutorial, then validate each section in the generated docs. Done means the specification has been manually reviewed and updated where needed, but the repository is marked decommissioned.

Written by the indexing model from the issue text.