add version staleness indicator

[ES-Alexander]

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

[InderParmar]

Hi, I am interested in working on this issue, can you please assign this issue to me. Thanks!

[ES-Alexander]

Hi @InderParmar, thanks for your interest in contributing! 😃

Happy to assign this to you.

Note that adding colours to the version selector may not make much sense until #3 is complete, but adding a banner or indicator should already be possible by checking whether /docs/latest or /docs/stable is in the config.base_url 🙂