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?
How about

```ruby
things.inject(urls) do |memo, thing|
  dest = thing.destination(destination)
  (memo[dest.downcase] ||= []) << dest
  memo
end
```