Add message to Maputnik URL to avoid 404 message for people unaware of MapLibre transition

[vknoppkewetzel]

It may be useful to have a message pop up for Maputnik users in the (now) old editor site URL.

I wasn't aware there was a transition to MapLibre (yay!) and when trying to dive back into stylework in the Maptunik editor I was confused about the editor not working and assumed there was work being done and kept circling back later (clearly there was!)

Maybe on https://maputnik.github.io/editor page just a simple "Maputnik has moved to MapLibre! head over to this URL instead: https://maplibre.org/maputnik "