docs(alert): add source reference for SeedExtendTrackExtractor

Author: mathieuouillon

reconstruction/alert/src/main/java/org/jlab/rec/alert/AI/SeedExtendTrackExtractor.java

@@ -9,6 +9,8 @@
  *  at a single threshold. Ports the {@code method="cc"} branch of
  *  {@code track-finding/gnn/inference.py::extract_tracks}, which is the
  *  extractor that gnn/evaluate.py uses.
+ * 
+ * Code from: <url> https://gitlab.com/MathieuOuillon/track-finding </url>
  */
 final class SeedExtendTrackExtractor {