Since gog doesn't have one I've tried to find it by myself, but with no success...
Diablo wiki has all texts from it, but I just can't find any scans of the actual manual.
Does it even exist?

There was a small manual in the cd case describing the story, monk and new spells.

If somebody steps forward with a hosting solution, I can try to make scans of the German Hellfire manual.

I suppose it would be appropriate to put it on archive.org