Add a Jekyll doctor warning for URLs that only differ by case

Those URLs are problematic on case-insensitive file systems because one of the URLs is overwritten by the other. Fixes #3035.

Note: I can't seem to find tests for doctor, I'm assuming there are none?
Sure, but this would make it inconsistent with `collect_urls()` above. I'd say consistency is more important than short code? Or should I change the other method too?