Added a delegate relation for the banking example

[aaguiarz]

Description

In banking scenarios, we hear very often that users want to share access for a specific bank account with other people (spouse, parents, children, associates).

I'm adding an example of how to model that scenario.

References

Review Checklist

• I have clicked on "allow edits by maintainers".
• I have added documentation for new/changed functionality in this PR or in a PR to openfga.dev [Provide a link to any relevant PRs in the references section above]
• The correct base branch is being used, if not main
• I have added tests to validate that the change in functionality is working as expected