doc-gen4 preview

Stream: lean4

[Henrik Böving]

We've made available a continuously deployed preview of doc-gen4's output on mathlib4 over here: https://leanprover-community.github.io/mathlib4_docs/ it's still far from feature complete but feel free to check it out and report on any bugs you find (that aren't yet in the tracker of course) over here https://github.com/leanprover/doc-gen4/issues