Add three changelog entries for new tutorials.

diff --git a/doc/news/changes/major/20210504Bangerth b/doc/news/changes/major/20210504Bangerth
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+New: The step-77 tutorial explores one of the mentioned extensions to step-15,
+namely implementing a proper step length control algorithm. It uses the KINSOL
+nonlinear solver package from the SUNDIALS library to incorporate a line
+search method into the incremental update for Newton's method.
+<br>
+(Wolfgang Bangerth, 2021/05/04)

diff --git a/doc/news/changes/major/20210506Jean-PaulPelteret b/doc/news/changes/major/20210506Jean-PaulPelteret
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+New: The step-71 tutorial demonstrates how automatic and symbolic differentiation
+can be leveraged to compute the derivatives of a coupled constitutive law.
+<br>
+(Jean-Paul Pelteret, 2021/05/06)

diff --git a/doc/news/changes/major/20210506Jean-PaulPelteret-2 b/doc/news/changes/major/20210506Jean-PaulPelteret-2
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+New: The step-72 tutorial revisits step-15 (that solves a nonlinear elliptic
+problem using Newton's method). In this variant, automatic differentiation
+is used either to linearize the residual, or to compute the full linear system
+from an energy functional.
+<br>
+(Jean-Paul Pelteret, 2021/05/06)