Clearly mark unmaintained distros separately from maintained distro in the website

[traversaro]

In https://robostack.github.io/, we are listing all available packages, but I think it would be useful to separate actually maintained distributions (noetic, humble, jazzy, kilted as of July 2025) from other distros that are not mantained anymore and are there for historical reference.

I guess this is a good task for anyone interested in starting contributing to robostack.