[Content]: Better outline and information on FileRoutes

📚 Subject area/topic

SolidStart, FileRoutes

📋 Page(s) affected (or suggested, for new content)

https://docs.solidjs.com/solid-start/building-your-application/routing#file-based-routing

📋 Description of content that is out-of-date or incorrect

Users have expressed concerns regarding how file-based routing is documented. There isn't enough detail that's leading them to making mistakes:

Hey people can you help me with the nested routing while using FileRoutes
I have everything configured but nested route is showing page  not found

From our Discord where similar reports have been made a couple of times. What might be valuable is outlining nested routes in a clearer and more concise manner with more examples.

🖥️ Reproduction in StackBlitz (if reporting incorrect content or code samples)

No response

Öffne den Abschnitt FileRoutes auf der verlinkten SolidStart-Routing-Seite und überprüfe, wie verschachtelte Routen derzeit erklärt werden. Präzisiere die Anleitung zum Routing mit verschachtelten Routen anhand knapper Beispiele und berücksichtige das gemeldete page-not-found-Szenario; die Arbeit ist abgeschlossen, wenn Benutzer die erwartete Dateistruktur und Konfiguration verstehen können.