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

[kbarros]

Some more motivation. As of Sunny 0.7.3, cloning the main branch only requires 8.2 MB of space:

git clone --single-branch -b main [email protected]:SunnySuite/Sunny.jl.git
du -sh Sunny.jl
# 8.2M	Sunny.jl

A full clone downloads the documentation contained in the gh-pages branch, which includes many Sunny releases. This takes 131 MB of space:

git clone [email protected]:SunnySuite/Sunny.jl.git
du -sh Sunny.jl
# 131M	Sunny.jl

Note that the docs are still in compressed form. Checking out gh-pages will decompress them into of 326 MB data:

cd Sunny.jl
git checkout gh-pages
du -sh .
# 326M	.