Improve layer checker description and examples

[Catsuko]

For the layer checker, the description focuses on enforcing dependencies only on layers below. However this isn't correct, layers are allowed from the same or lower layers (as seen in the violation message):

Packs in a lower layer may not access packs in a higher layer. See the layers in packwerk.yml. Current hierarchy:

I've reworded the description slightly and adjusted the example to better demonstrate this. Let me know what you think 🙏

[martinemde]

Thank you for your help making the documentation clearer. 3 layers helps explain the behavior better.