Check that the alert candidate is not already in an alert block or nested within other elements.

Author: snnz

src/Markdig/Extensions/Alerts/AlertInlineParser.cs

@@ -29,7 +29,8 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
         // We expect the alert to be the first child of a quote block. Example:
         // > [!NOTE]
         // > This is a note
-        if (processor.Block is not ParagraphBlock paragraphBlock || paragraphBlock.Parent is not QuoteBlock quoteBlock || paragraphBlock.Inline?.FirstChild != null)
+        if (processor.Block is not ParagraphBlock paragraphBlock || paragraphBlock.Parent is not QuoteBlock quoteBlock || paragraphBlock.Inline?.FirstChild != null
+            || quoteBlock is AlertBlock || quoteBlock.Parent is not MarkdownDocument)
         {
             return false;
         }