Warning Box to all pages of old versions

[alexmorley]

Is there an easy way to add a warning box to all documentation for a set of versions?

Ref: JuliaLang/julia#22936

[odow]

I'm doing a triage of open documenter issues and closing issues that are stale. If I've made a mistake, please re-open

Closing because this now happens. For things that are not stable, you get: