Exclude secondary Markdown output files from sitemap (#7018)

Fixes #7017

Author: parlough

tool/dash_site/lib/src/commands/build.dart

@@ -41,6 +41,8 @@ final class BuildSiteCommand extends Command<int> {
         // those specified by jaspr_cli.
         '--no-managed-build-options',
         '--sitemap-domain=https://dart.dev',
+        // Exclude secondary Markdown output files from sitemap.
+        r'--sitemap-exclude=\.html\.md$',
         '--dart-define=PRODUCTION=$productionRelease',
       ],
       workingDirectory: 'site',