feat(download/index): add instructions for installing Lean using PPA and Homebrew
feat(download/index): add instructions for installing Lean using PPA and Homebrew

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