Add an example of admonition with warning style

[paugier]

I didn't find this useful tip (change the class of an admonition) in the documentation. See https://jupyterbook.org/en/stable/interactive/hiding.html?highlight=admonition#toggle-admonition-content-with-dropdowns and https://jupyterbook.org/en/stable/advanced/html.html?highlight=admonition#an-example-custom-admonitions for related but different things.

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Codecov Report

Base: 91.39% // Head: 91.39% // No change to project coverage :thumbsup:

Coverage data is based on head (cb229e8) compared to base (5e5f629). Patch has no changes to coverable lines.

:exclamation: Current head cb229e8 differs from pull request most recent head f95183a. Consider uploading reports for the commit f95183a to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1834   +/-   ##
=======================================
  Coverage   91.39%   91.39%           
=======================================
  Files           7        7           
  Lines         686      686           
=======================================
  Hits          627      627           
  Misses         59       59           

Flag	Coverage Δ
pytests	91.39% <ø> (ø)

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[paugier]

Hi, I rebased on master and I think it could be ready to be merged :-)

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