Building lean4_docs like mathlib4_docs

Stream: lean4

[Siddharth Bhat]

I spun up a small repo to generate lean4 docs with doc-gen4, bollu.github.io/lean4_docs [The github repo is https://github.com/bollu/lean4_docs]. I feel it'd be useful for it to be under leanprover/lean4_docs`, since we can then write a deploy script that updates the latest version of the docs. What do others feel?

[Mac]

I think this sounds like a good idea. Maybe it could somehow be combine with @Xubai Wang's Reservoir package index?