add link to lean-mode slides
add link to lean-mode slides

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