Include page title and description in md output (#12639)

Includes the page title and description (if present) in the secondary
Markdown output for each page.

**Staged example:**
https://flutter-docs-prod--pr12639-feat-header-in-markdown-out-0w19b91b.web.app/ui/layout/tutorial/index.html.md

Author: parlough

site/lib/main.dart

@@ -64,7 +64,26 @@ Component get _docsFlutterDevSite => ContentApp.custom(
     components: _embeddableComponents,
     layouts: const [DocLayout(), TocLayout(), CatalogPageLayout()],
     theme: const ContentTheme.none(),
-    secondaryOutputs: [const RobotsTxtOutput(), MarkdownOutput()],
+    secondaryOutputs: [
+      const RobotsTxtOutput(),
+      MarkdownOutput(
+        createHeader: (page) {
+          final header = StringBuffer();
+          if (page.data.page['title'] case final String title
+              when title.isNotEmpty) {
+            header.writeln('# $title');
+
+            if (page.data.page['description'] case final String description
+                when description.isNotEmpty) {
+              header.writeln();
+              header.writeln('> $description');
+            }
+          }
+
+          return header.toString();
+        },
+      ),
+    ],
   ),
 );
 

site/lib/src/components/common/tabs.dart

@@ -8,11 +8,11 @@ import 'package:jaspr_content/jaspr_content.dart';
 import '../../util.dart';
 
 /// A tabs component where children tabs can be switched between by the user.
-class DashTabs implements CustomComponent {
+class DashTabs extends CustomComponent {
   static int _currentTabWrapperId = 0;
   static int _currentTabId = 0;
 
-  const DashTabs();
+  const DashTabs() : super.base();
 
   @override
   Component? create(Node node, NodesBuilder builder) {

site/pubspec.yaml

@@ -9,11 +9,11 @@ environment:
 dependencies:
   build: ^4.0.2
   collection: ^1.19.1
-  crypto: ^3.0.6
+  crypto: ^3.0.7
   html: ^0.15.6
   http: ^1.5.0
   jaspr: ^0.21.6
-  jaspr_content: ^0.4.2
+  jaspr_content: ^0.4.3
   # Used as our template engine.
   liquify: ^1.3.1
   markdown: ^7.3.0
@@ -32,9 +32,9 @@ dev_dependencies:
       path: pkgs/analysis_defaults
       ref: f91ed8ecef6a0b31685804fe4102b25fda021460
   build_runner: ^2.10.1
-  build_web_compilers: ^4.3.0
+  build_web_compilers: ^4.4.0
   jaspr_builder: ^0.21.6
-  sass: ^1.93.2
+  sass: ^1.93.3
   sass_builder: ^2.4.0
 
 jaspr: