Update index.html with github ribbon
Update index.html with github ribbon

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