Reflect that Stamen maps tiles are now hosted by Stadia Maps

[ianthetechie]

See http://maps.stamen.com/stadia-partnership/.

Basically you'd just need to change most of the URL prefixes I'm not sure how you want to communicate this exactly, but I'm happy to provide assistance if desired.

One thing to note is that you should probably not directly link to the tiles, since they require either domain-level or API key auth. I'd suggest just committing a single tile of each to the repo as an example, or maybe linking to the OSM wiki thumbnails (not sure of their link policy though so check on that!): https://wiki.openstreetmap.org/wiki/List_of_OSM-based_services