Implement support for single-file Quadlet configurations, allowing users to define multiple related units (containers, volumes, networks) in a single file. This will simplify Quadlet management and improve user experience.

This story involves:

• Designing and implementing single-file Quadlet format
• Supporting multiple unit definitions within a single file
• Ensuring proper parsing and validation of single-file Quadlets
• Creating a design document outlining the single-file format
• Updating documentation to describe single-file Quadlet usage
• Adding tests to validate single-file Quadlet functionality