Fix API key required for CartoDB map tiles

CartoDB now requires an API key, otherwise they display "API KEY REQUIRED" on their map tiles:
https://docs.bikenetkit.org/GrowBikeNet/mwe/

We cannot have tiles API handling in the public docs (especially mwe or basic usage which have to be as minimal as possible), so if docs are not possible without that we need to switch to another tile provider. Figure out with @MoritzStefaner what is best, so the docs maps are as close as possible to the viz maps.