Landing page for failed FGData auto-download

Related to various failures we see in the wild: flightgear#3168

While we can improve various cases and fallbacks, the ultimate (and easiest) fallback is to document how to get the files manually. So we need a landing page, probably keyed to the stable version, that I can reference from the 'setup root' dialog, and point people at.

This probably just needs to point people at https://download.flightgear.org/release-2024.1/

and tell them how to extract a TXZ on their system?