API-Tests: Add links to Strapi documentation on populating relations to the `bruno-test.md` file

When making the tests I noticed that Strapi doesn't populate relation fields in API responses by default. It was hard to find docs explaining how to do this because Strapi forgot to update the doc page from v4 to v5 so I think we should add a link to how to the doc page to make it easier for everyone else down the line.

https://docs-v4.strapi.io/dev-docs/api/rest/guides/understanding-populate