feat: use GitHub's markdown alerts for warnings in README

[ayhon]

Relatively recently GitHub has added support for adding a "callout" element in markdown files (source). I was reading the README and thought it could be something the project might be interested in using, since it has a couple of warnings in the README.

Instead of

⚠️ Warning! ⚠️ : Critical content demanding immediate user attention due to potential risks.

it is shown like

Warning

Critical content demanding immediate user attention due to potential risks.

which in my opinion stands out more, which is why I thought you might find it interesting.

This is achieved with the following markdown

> [!WARNING]
> Critical content demanding immediate user attention due to potential risks.

[eclipse-zenoh-bot]

@ayhon If this pull request contains a bugfix or a new feature, then please consider using Closes #ISSUE-NUMBER syntax to link it to an issue.

[JEnoch]

Thanks for your contribution!
That's indeed more catchy and nicer to read 😃