Two components of the boundary of \(\text \textbf{I}_3\). (Deux composantes du bord de \(\text \textbf{I}_3\)) Moduli of curves. CIMAT Guanajuato, Mexico 2016. Lecture notes of a CIMPA-ICTP school, Guanajuato, Mexico, February 22 -- March 4, 2016 