Adding a Map Key for the stylesheet on osm.org

[skorasaurus]

I noticed that only the default stylesheet.

Would be a nice enhancement to have a map key (Aka: legend) for our stylesheet.

I'm including some links to code that are likely relevant. ref: https://github.com/openstreetmap/openstreetmap-website/commit/815c98c977831a9a7f84dca49186f5eea7b4a283