Add AI top-level section and restructure existing AI docs (#13264)

Adds top level AI header and reorganizes our current AI landing page and
AI content.

## Presubmit checklist

- [X] If you are unwilling, or unable, to sign the CLA, even for a
_tiny_, one-word PR, please file an issue instead of a PR.
- [X] If this PR is not meant to land until a future stable release,
mark it as draft with an explanation.
- [X] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style)—for example, it doesn't
use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first-person
pronouns).
- [X] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
  of 80 characters or fewer.

Author: lamek

site/lib/src/components/layout/header.dart

@@ -62,6 +62,11 @@ class DashHeader extends StatelessComponent {
             label: 'Learn',
             isActive: activeEntry == ActiveNavEntry.learn,
           ),
+          _NavItem(
+            href: '/ai/create-with-ai',
+            label: 'AI',
+            isActive: activeEntry == ActiveNavEntry.ai,
+          ),
           const _NavItem(
             href: 'https://api.flutter.dev',
             label: 'Reference',

site/lib/src/components/layout/sidenav.dart

@@ -43,6 +43,12 @@ final class DashSideNav extends StatelessComponent {
           iconId: 'play_lesson',
           active: activeEntry == ActiveNavEntry.learn,
         ),
+        _TopNavItem(
+          href: '/ai/create-with-ai',
+          label: 'AI',
+          iconId: 'auto_awesome',
+          active: activeEntry == ActiveNavEntry.ai,
+        ),
         const _TopNavItem(
           href: 'https://api.flutter.dev',
           label: 'Reference',

site/lib/src/layouts/dash_layout.dart

@@ -157,7 +157,12 @@ ga('send', 'pageview');
     final bodyClass = pageData['bodyClass'] as String?;
     final pageUrl = page.url.startsWith('/') ? page.url : '/${page.url}';
 
+    final pageSidenav = pageData['sidenav'] as String? ?? defaultSidenav;
     final sideNavEntries = switch (page.data['sidenav']) {
+      _ when pageSidenav == 'ai' => switch (page.data['ai']) {
+        final List<Object?> sidenavData => navEntriesFromData(sidenavData),
+        _ => null,
+      },
       final List<Object?> sidenavData => navEntriesFromData(sidenavData),
       _ => null,
     };

site/lib/src/utils/active_nav.dart

@@ -7,11 +7,15 @@ ActiveNavEntry activeNavEntry(String pageUrlPath) {
     return ActiveNavEntry.learn;
   }
 
+  if (pageUrlPath.startsWith('/ai')) {
+    return ActiveNavEntry.ai;
+  }
   return ActiveNavEntry.home;
 }
 
 enum ActiveNavEntry {
   home,
   learn,
+  ai,
   // reference,
 }

src/content/ai/ai-rules.md

@@ -1,5 +1,6 @@
 ---
 title: AI rules for Flutter and Dart
+sidenav: ai
 shortTitle: AI rules
 description: >-
   Learn how to add AI rules to tools that accelerate your

src/content/ai/ai-toolkit/chat-client-sample.md

@@ -1,5 +1,6 @@
 ---
 title: Chat client sample
+sidenav: ai
 description: >
   Learn about the chat client sample included in the AI Toolkit.
 prev:

src/content/ai/ai-toolkit/custom-llm-providers.md

@@ -1,5 +1,6 @@
 ---
 title: Custom LLM providers
+sidenav: ai
 description: >
   How to integrate with other Flutter features.
 prev:

src/content/ai/ai-toolkit/feature-integration.md

@@ -1,5 +1,6 @@
 ---
 title: Feature integration
+sidenav: ai
 description: >
   How to integrate with other Flutter features.
 prev:

src/content/ai/ai-toolkit/index.md

@@ -1,5 +1,6 @@
 ---
 title: Flutter AI Toolkit
+sidenav: ai
 shortTitle: AI Toolkit
 description: >
   Learn how to add the AI Toolkit chatbot

src/content/ai/ai-toolkit/user-experience.md

@@ -1,5 +1,6 @@
 ---
 title: User experience
+sidenav: ai
 description: >
   How the user will experience the AI Toolkit in your app.
 prev: