Add go_router 15 migration guide #166576
Description
Document Link
What problem are you solving?
Based on https://www.w3.org/TR/WD-html40-970708/htmlweb.html, the URL should be case-sensitive. But that is not the case for go_router.
#162487 highlighted the issue and the inconsistent behavior.
flutter/packages#8992 makes go_router's URL case-sensitive, which is a breaking change.
The migration guide has been added int flutter/website#11867