add information on where to place the code mentioned in custom routing section

[jeyj0]

https://ihp.digitallyinduced.com/Guide/routing.html#custom-routing is missing a guide on where to place the necessary code. It's only included in the static routes section of the guide.

Origin on slack: https://ihpframework.slack.com/archives/C01DQE0F4F8/p1636695628207600