Lean doc preview

Stream: mathlib4

[Scott Morrison]

@Bryan Gin-ge Chen's github lean doc preview tool seems to more or less work when given mathlib4 URLs, so I'm going to continue linking from it https://leanprover-community.github.io/contribute/doc.html unless someone advises me otherwise.

It would be nice if the example URLs on that page were changed to point to mathlib4 PRs and commits, and if entering just a PR number in the search box looked up that PR on mathlib4 rather than mathlib, but I guess these aren't essential.