feat(download): add debian.png
feat(download): add debian.png

fpvandoorn/leanprover.github.io,leanprover/leanprover.github.io,soonhokong/leanprover.github.io,leanprover/leanprover.github.io,soonhokong/leanprover.github.io,avigad/leanprover.github.io,fpvandoorn/leanprover.github.io,fpvandoorn/leanprover.github.io,soonhokong/leanprover.github.io,avigad/leanprover.github.io,leanprover/leanprover.github.io,avigad/leanprover.github.io