docs: hierarchy of domains

[vinzscam]

Hey, I just made a Pull Request!

Updated the Backstage System Model Diagram to reflect the changes in https://github.com/backstage/backstage/pull/17125

:heavy_check_mark: Checklist

• [ ] A changeset describing the change and affected packages. (more info)
• [x] Added or updated documentation
• [ ] Tests for new functionality and regression tests for bug fixes
• [ ] Screenshots attached (for UI changes)
• [x] All your commits have a Signed-off-by line in the message. (more info)

[github-actions[bot]]

Thank you for contributing to Backstage! The changes in this pull request will be part of the 1.28.0 release, scheduled for Tue, 18 Jun 2024.