Expand the `defining-relationships.md` skill to include a section to show how to create programmatic queries for relationships

[kylebernhardy]

In asking the agent regarding best practices it noted a gap in how to create programmatic queries for related tables. It instead suggested a fallback to how REST works. In order to better inform the Agent we should add a section showing example resource code with a query that includes relationships. For reference: https://docs.harperdb.io/docs/reference/resources#select