feat(admonition): add support for content-only alerts without titles (#683)

* feat(admonition): add support for content-only alerts without titles

* fix(admonition): improve title-only alert handling and styling

apps/test/content/posts/alert-and-admonition-test.md

@@ -39,7 +39,7 @@ This article comprehensively tests the **Alerts** Markdown syntax extension and
 > [!NOTE] FixIt
 > A clean, elegant but advanced Hugo theme.
 
-> [!TIP] This is a title-only alert
+> [!TIP] This is a title-only alert.
 
 ### Foldable Alerts
 
@@ -59,6 +59,11 @@ Alerts can be nested in multiple levels:
 > > [!todo] Yes! They can be nested.
 > > > [!example] You can even use multiple layers of nesting.
 
+### Content-only Alerts
+
+> [!TIP]~
+> This is a content-only alert without a title.
+
 ### Supported types
 
 > [!note]+