DOCS: Add an example of admonition with warning style (#1834)

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.