Support GFM admonitions

### Context

See https://github.com/executablebooks/MyST-Parser/issues/845. Basically this:

```md
> [!NOTE]
> This is a note
```

Looks like this on GitHub:

> [!NOTE]
> This is a note


It was suggested in that issue that the implementation should start here.

### Proposal

This is a feature of GFM, so it should be supported.

### Tasks and updates

_No response_