docs: add a recipe for custom error handling

[Slavaqq]

Summary of Changes

This PR adds a recipe for handling a custom error. It was created during EuroPython 2026 sprints, and it is draft.

What needs to be improve:

• write tests for the recipe
• enrich the description in the documentation

Please let me know if this is a good start and what else I need to improved.
I would like to continue with this issue if it is possible.

Related Issues

#2692

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (46ab186) to head (3071ede).

Additional details and impacted files

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##            master     #2701   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           64        64           
  Lines         7987      7987           
  Branches      1105      1105           
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  Hits          7987      7987           

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