Outdated script for preview docs

[fredrikekre]

The outdated warning script is not working for e.g. https://fredrikekre.github.io/Literate.jl/previews/PR184/ because we look for versions.js in the wrong place (one extra subfolder with /previews). Would be good to make this work.

Instead of fixing the link to versions.js I think that we could just special case the path /previews/PR## pattern and add a "preview warning" and noindex.