[CI] Add workflow to clean up docs previews

[giordano]

No description provided.

[giordano]

For future reference, I manually cleaned up all old previews with the following script:

using GitHub: GitHub

auth = GitHub.authenticate(ENV["GITHUB_TOKEN"])
repo = GitHub.repo("EnzymeAD/Reactant.jl"; auth)
pull_requests = GitHub.pull_requests(repo; auth)
pr_numbers = [pr.number for pr in pull_requests[1]]

previews_dir = joinpath(Base.DEPOT_PATH[1], "dev", "Reactant", "previews")

if !isdir(previews_dir)
    error("$(previews_dir) does not exist, fix the location of the previews directory")
end

for preview in readdir(previews_dir)
    preview_number = parse(Int, replace(preview, "PR" => ""))
    if preview_number ∉ pr_numbers
        run(`git -C $(previews_dir) rm -rf PR$(preview_number)`)
    end
end