Add alert blocks in markdown (#29121)

- Follows https://github.com/go-gitea/gitea/pull/21711
- Closes https://github.com/go-gitea/gitea/issues/28316

Implement GitHub's alert blocks markdown feature

Docs:
-
https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#alerts
- https://github.com/orgs/community/discussions/16925

### Before

![image](14f7b02a-5de5-4fd0-8437-a055dadb31f2)

### After

![image](ed06a869-e545-42f1-bf25-4ba20b1be196)

## ⚠️ BREAKING ⚠️

The old syntax no longer works

How to migrate:

If you used
```md
> **Note** My note
```

Switch to
```md
> [!NOTE]
> My note
```

---------

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

modules/markup/markdown/ast.go

@@ -182,12 +182,7 @@ func IsColorPreview(node ast.Node) bool {
 	return ok
 }
 
-const (
-	AttentionNote    string = "Note"
-	AttentionWarning string = "Warning"
-)
-
-// Attention is an inline for a color preview
+// Attention is an inline for an attention
 type Attention struct {
 	ast.BaseInline
 	AttentionType string