Find an alternative for displaying contours of signed distance field functions

[ferrolho]

The Contours example shown in the Readme.md is no longer working. This is because the SignedDistanceField function that the example relies on is no longer available after switching from the now-deprecated GeometryTypes.jl to GeometryBasics.jl. I will add a warning to the Readme.md until we found a working solution.

For related threads, see:

• https://github.com/JuliaGeometry/GeometryBasics.jl/issues/72
• https://discourse.julialang.org/t/which-package-function-for-signed-distance-fields/43487/2