Enable alert blocks in Markdig markdown converter

Added the UseAlertBlocks extension to the Markdig pipeline to support alert block syntax in markdown rendering.

modules/docs/src/Volo.Docs.Web/Markdown/MarkDigMarkdownConverter.cs

@@ -19,6 +19,7 @@ namespace Volo.Docs.Markdown
               .UseGridTables()
               .UsePipeTables()
               .UseHighlightedCodeBlocks()
+              .UseAlertBlocks()
               .Build();
         }