Host old docs versions on `gh-pages`

[kbarros]

Our Github actions automatically build and deploy the main docs on https://sunnysuite.github.io/Sunny.jl/dev/

It should be possible to build docs for the "release" versions of Sunny. Relevant links are:

• https://documenter.juliadocs.org/stable/man/hosting/#Documentation-Versions
• https://documenter.juliadocs.org/stable/lib/public/#Documenter.deploydocs

Per @Lazersmoke

This is a task that should probably be done once, manually, per version. In particular, we don't want to rebuild historical versions on each commit. The current automated workflow should be thought of as simply keeping the dev folder in the gh-pages branch up to date. Then, someone can manually call makedocs and deploydocs after checking out each version we want to have the stable docs for, with the versions kwarg set to just that specific version by name (i.e. someone just needs to generate the appropriate docs and put them on gh-pages branch, manually by a commit or by manually calling deploydocs)

It seems like there might still be some details to figure out. Help wanted!

[kbarros]

Abandoning this for now. Our interim solution is to link to the docs on JuliaHub, which are built and hosted for each Sunny release: https://juliahub.com/ui/Packages/General/Sunny