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?
As @pathawks correctly mentions, the current FS doesn't matter as the site might be used on a different FS too. This is exactly what happened to us and made me raise the issue: one dev worked on Linux and didn't notice URLs only differed by case. Then another dev on OSX wanted to work on the site, but the page got overwritten.