docs: Custom Handler for non-file-based Routing

[garysm]

Description
The docs do not currently state how to implement a custom router if file-based routing is not desired. This was requested in #467 and #530.

I provided an example based on shelf_router in #530, but any other router would be fine too.

Requirements

• Provide a clear example on how to implement a non-file-based router
• Provide references in Routing/Middleware/Custom Server Entrypoint sections

[renancaraujo]

Tracking this on #467, thanks for opening an issue