Don't use <br /> in alert block (#29650)

- Follows #29121

When I implemented alert blocks I was always testing the markdown in
issue comments. I used `<br />` for line breaks and it looked good. I
have since learned that the markdown on README files doesn't allow these
tags. So a comment with

```md
> [!NOTE]
> If you're interested in using our APIs, we have experimental support with [documentation](https://try.gitea.io/api/swagger).
```

looked like this in a comment

![image](https://github.com/go-gitea/gitea/assets/20454870/96b1de01-2c87-4d4f-83dd-98192b83e9d0)
but looked like this in a README

![image](https://github.com/go-gitea/gitea/assets/20454870/474b636d-dd7a-4b7f-ba27-643803c71aa3)

So I changed how we render the alert block by having the alert itself
have a dedicated paragraph, so line breaks happen naturally between
paragraphs.

# Before

![image](https://github.com/go-gitea/gitea/assets/20454870/474b636d-dd7a-4b7f-ba27-643803c71aa3)

![image](https://github.com/go-gitea/gitea/assets/20454870/167a8d37-9a44-4479-9340-5dc80347b595)

# After

![image](https://github.com/go-gitea/gitea/assets/20454870/2f99fec0-98ff-4ba8-97fe-b4567041ae79)

![image](https://github.com/go-gitea/gitea/assets/20454870/ffdeae11-fb06-4d00-b497-eae135f0d7ad)

---------

Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>

Author: yardenshoham

modules/markup/markdown/goldmark.go

@@ -219,21 +219,18 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 			v.SetAttributeString("class", []byte("gt-py-3 attention attention-"+attentionType))
 
 			// create an emphasis to make it bold
+			attentionParagraph := ast.NewParagraph()
 			emphasis := ast.NewEmphasis(2)
 			emphasis.SetAttributeString("class", []byte("attention-"+attentionType))
-			firstParagraph.InsertBefore(firstParagraph, firstTextNode, emphasis)
 
 			// capitalize first letter
 			attentionText := ast.NewString([]byte(strings.ToUpper(string(attentionType[0])) + attentionType[1:]))
 
-			// replace the ![TYPE] with icon+Type
+			// replace the ![TYPE] with a dedicated paragraph of icon+Type
 			emphasis.AppendChild(emphasis, attentionText)
-			for i := 0; i < 2; i++ {
-				lineBreak := ast.NewText()
-				lineBreak.SetSoftLineBreak(true)
-				firstParagraph.InsertAfter(firstParagraph, emphasis, lineBreak)
-			}
-			firstParagraph.InsertBefore(firstParagraph, emphasis, NewAttention(attentionType))
+			attentionParagraph.AppendChild(attentionParagraph, NewAttention(attentionType))
+			attentionParagraph.AppendChild(attentionParagraph, emphasis)
+			firstParagraph.Parent().InsertBefore(firstParagraph.Parent(), firstParagraph, attentionParagraph)
 			firstParagraph.RemoveChild(firstParagraph, firstTextNode)
 			firstParagraph.RemoveChild(firstParagraph, secondTextNode)
 			firstParagraph.RemoveChild(firstParagraph, thirdTextNode)