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?
This isn't necessary – the `#inject` call creates the `memo` based on `urls` already. Ideally we'd have:

```ruby
def urls_only_differ_by_case(site)
  urls = case_insensitive_urls(site.pages + site.posts, destination)
  # etc
end

def case_insensitive_urls(things, destination)
  things.inject(Hash.new) do |memo, thing|
    dest = thing.destination(destination)
    (memo[dest.downcase] ||= []) << dest
    memo
  end
end
```

The `inject` call returns the `memo` object, and we can do it just once for all pages and posts rather than mutating things.