add version staleness indicator

The version selector tells you which version you are viewing docs for, but for fixed release numbers needs to be clicked on to get a sense of whether that release is recent or old, and for latest/stable it may not be obvious that the other is available to view.

It would be helpful to have some indicator/message/banner as part of the page template that specifies whether the version being looked at is up to date, or if there are more recent docs that should be referred to.

As part of that (and building on #3), it may be worth colouring the displayed version(s) in the selector with a colour range representing the staleness of versions (e.g. blue or purple for latest, green for stable, and orange to red for old releases).

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Inherits ideas from bluerobotics/ardusub-zola#58

Start with the page template and version selector, then review the related ideas in #3 and bluerobotics/ardusub-zola#58. Define how fixed releases, latest, and stable should indicate recency and availability of newer documentation. Done means the page communicates version staleness and the selector provides a clear visual distinction across versions.

Written by the indexing model from the issue text.