Simplify expression for tangent in hyperelasticity tutorial

[KnutAM]

• Simplify expression for tangent
• Use new admonition syntax
• Add test for simplified equation

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.72%. Comparing base (0e26c10) to head (370802c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #852   +/-   ##
=======================================
  Coverage   93.72%   93.72%           
=======================================
  Files          39       39           
  Lines        6008     6008           
=======================================
  Hits         5631     5631           
  Misses        377      377           

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