Add example use case documentation

[Dev-Owl]

Add example use case documentation, this can be based on the example project but should also cover:

• [ ] Important things to know
• [ ] - Register all traits
• [ ] - Why is the realm null for some nodes?
• [ ] What default resource are included in backbone
• [ ] How to use the input system with multiple realms
• [ ] How to use just part's of the input system

Please extend the list with further points.