[BUG] Center icon and callout text

- Wrap the icon and callout into a seperate `<p>` which has `display:
flex; align-items: center` set. To center the icon with the callout text.
- Resolves #3006

(cherry picked from commit 6ffae461d4)

modules/markup/markdown/callout/github.go

@@ -72,6 +72,7 @@ func (g *GitHubCalloutTransformer) Transform(node *ast.Document, reader text.Rea
 
 			// create an emphasis to make it bold
 			attentionParagraph := ast.NewParagraph()
+			attentionParagraph.SetAttributeString("class", []byte("attention-title"))
 			emphasis := ast.NewEmphasis(2)
 			emphasis.SetAttributeString("class", []byte("attention-"+attentionType))
 			firstParagraph.InsertBefore(firstParagraph, firstTextNode, emphasis)