doc: add example on how to use a helper function to assert a custom struct

[haraldmaida]

Often the easiest way to assert a custom type is to write a helper function that asserts each part of the type reusing existing assertions.

Added the example assertion_function.rs to demonstrate how to write a helper function to assert the fields of a custom struct reusing existing assertions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.95%. Comparing base (1b7603d) to head (f1f3572).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #67   +/-   ##
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  Coverage   93.95%   93.95%           
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  Files          32       32           
  Lines        4463     4463           
  Branches     4463     4463           
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  Hits         4193     4193           
  Misses        233      233           
  Partials       37       37           

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