Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
Yes, but they have a heading because that's the only way to identify what in the alert is heading of that alert. With a details/summary, that's already defined.

section = details
h3 = summary

I'm not against adding it, I'm just saying it's not about being against or for. That should be up to the person designing the content. There's plenty of reasons to both include or exclude an alert from an outline.

Such possibilities should be made explicit. I'm saying we'll add examples with both and explain the use case for those less familiar with HTML semantics.