[doc] Document how to do `build.dart` that uses system libraries

[dcharkes]

It should output assets with AssetSystemPath.

/// Asset is avaliable on the system `PATH`.
///
/// [uri] only contains a file name.
abstract class AssetSystemPath implements AssetPath {
  Uri get uri;

  factory AssetSystemPath(Uri uri) = model.AssetSystemPath;
}

We should also document how to set up your path correctly.

Also, we should double check if only using the file name + path works in all cases or if we need to support absolute paths as well.

The dependencies in the BuildOutput should contain build.dart (or one will always get a cached list of assets).