Deploy docs to separate repo

[kbarros]

Our docs images are high res and quite large. Hosting docs for all releases will be quite large -- I could imagine this growing to ~ GB, which will slow down clones of Sunny. We could follow Meshes.jl and create a separate repo to store the gh-pages for docs deployment:
https://github.com/JuliaGeometry/Meshes.jl/blob/master/docs/make.jl

[kbarros]

This is now described in the Documenter docs: https://documenter.juliadocs.org/stable/man/hosting/#Out-of-repo-deployment