proposal: extend admonition with tagline

### Problem

Some people have custom HTML to create admonitions with extra taglines.

Came across this here:
https://github.com/kubernetes-sigs/kubebuilder/pull/5459

### Proposed Solution

We can simply add text to the admonition block syntax:

```
> ![NOTE For fun]
>
> Head to the party!
```

### Notes

_No response_