Add example style without API key requirement

[m0nac0]

For testing and demonstration purposes (and the CI) it would be good to have an example style with a tile source that doesn't require an API key.

Mapbox tiles are really good, but require an API key.

The tile source from the custom style (example/assets/style.json) in our example app is broken anyways, so we could replace that style file with an example style from Maplibre.
Possible tile-source: https://github.com/maplibre/demotiles