DOC add new reference for unsupervised feature selection tutorial

Author: MatthewSZhang

doc/unsupervised.rst

@@ -31,8 +31,13 @@ matrix :math:`X`. See the example below.
 
 .. rubric:: References
 
+* `"Manifold learning-based unsupervised feature selection for structural health
+  monitoring" <https://mrforum.com/wp-content/uploads/open_access/9781644903513/9.pdf>`_
+  Wang, T., & Sun, L.
+  Materials Research Proceedings, 50, 82-89, (2025).
+
 * `"Automatic Selection of Optimal Structures for Population-Based
   Structural Health Monitoring" <https://doi.org/10.1007/978-3-031-34946-1_10>`_
-  Wang, T., Worden, K., Wagg, D.J., Cross, E.J., Maguire, A.E., Lin, W.
+  Wang, T., Worden, K., Wagg, D.J., Cross, E.J., Maguire, A.E., & Lin, W.
   In: Conference Proceedings of the Society for Experimental Mechanics Series.
   Springer, Cham. (2023).