Confirm that Scalingo buildpacks conform to the Cloud

In [this page](https://github.com/Scalingo/documentation/edit/master/src/_posts/platform/deployment/buildpacks/2000-01-01-custom.md) in the documentation, please somehow refer to https://buildpacks.io, and probably more specifically to the "[Buildpack Interface Specification](https://github.com/buildpacks/spec/blob/main/buildpack.md)".