Update Flutter desktop doc URL in error message (#96559)

When desktop support is not present in an existing project, certain
flutter tool commands raise an error that direct the user to
documentation on how to add desktop support to an existing Flutter
project. In a recent revamp of the webside, the URL was very slightly
changed (flutter.dev -> docs.flutter.dev).

This updates the error message to output the new URL.

Issue: https://github.com/flutter/flutter/issues/94398
diff --git a/packages/flutter_tools/lib/src/linux/build_linux.dart b/packages/flutter_tools/lib/src/linux/build_linux.dart
index 49f14a1..81c86d5 100644
--- a/packages/flutter_tools/lib/src/linux/build_linux.dart
+++ b/packages/flutter_tools/lib/src/linux/build_linux.dart
@@ -38,7 +38,7 @@
   target ??= 'lib/main.dart';
   if (!linuxProject.cmakeFile.existsSync()) {
     throwToolExit('No Linux desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Linux support to a project.');
   }
 
diff --git a/packages/flutter_tools/lib/src/macos/build_macos.dart b/packages/flutter_tools/lib/src/macos/build_macos.dart
index 335f634..c217750 100644
--- a/packages/flutter_tools/lib/src/macos/build_macos.dart
+++ b/packages/flutter_tools/lib/src/macos/build_macos.dart
@@ -31,7 +31,7 @@
 }) async {
   if (!flutterProject.macos.xcodeWorkspace.existsSync()) {
     throwToolExit('No macOS desktop project configured. '
-      'See https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'See https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding macOS support to a project.');
   }
 
diff --git a/packages/flutter_tools/lib/src/windows/build_windows.dart b/packages/flutter_tools/lib/src/windows/build_windows.dart
index 1d7a269..6422d39 100644
--- a/packages/flutter_tools/lib/src/windows/build_windows.dart
+++ b/packages/flutter_tools/lib/src/windows/build_windows.dart
@@ -32,7 +32,7 @@
   if (!windowsProject.cmakeFile.existsSync()) {
     throwToolExit(
       'No Windows desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Windows support to a project.');
   }
 
@@ -124,7 +124,7 @@
   if (!windowsProject.existsSync()) {
     throwToolExit(
       'No Windows UWP desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Windows support to a project.',
     );
   }
diff --git a/packages/flutter_tools/test/commands.shard/hermetic/build_linux_test.dart b/packages/flutter_tools/test/commands.shard/hermetic/build_linux_test.dart
index 9714f74..0d73157 100644
--- a/packages/flutter_tools/test/commands.shard/hermetic/build_linux_test.dart
+++ b/packages/flutter_tools/test/commands.shard/hermetic/build_linux_test.dart
@@ -114,7 +114,7 @@
     expect(createTestCommandRunner(command).run(
       const <String>['build', 'linux', '--no-pub']
     ), throwsToolExit(message: 'No Linux desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Linux support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => linuxPlatform,
diff --git a/packages/flutter_tools/test/commands.shard/hermetic/build_macos_test.dart b/packages/flutter_tools/test/commands.shard/hermetic/build_macos_test.dart
index febf951..4e5cc45 100644
--- a/packages/flutter_tools/test/commands.shard/hermetic/build_macos_test.dart
+++ b/packages/flutter_tools/test/commands.shard/hermetic/build_macos_test.dart
@@ -131,7 +131,7 @@
     expect(createTestCommandRunner(command).run(
       const <String>['build', 'macos', '--no-pub']
     ), throwsToolExit(message: 'No macOS desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding macOS support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => macosPlatform,
diff --git a/packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart b/packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart
index 1244d02..a85bec4 100644
--- a/packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart
+++ b/packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart
@@ -159,7 +159,7 @@
     expect(createTestCommandRunner(command).run(
       const <String>['windows', '--no-pub']
     ), throwsToolExit(message: 'No Windows desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Windows support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => windowsPlatform,
@@ -489,7 +489,7 @@
     expect(createTestCommandRunner(command).run(
       const <String>['winuwp', '--no-pub']
     ), throwsToolExit(message: 'No Windows UWP desktop project configured. See '
-      'https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
+      'https://docs.flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-app '
       'to learn about adding Windows support to a project.'));
   }, overrides: <Type, Generator>{
     Platform: () => windowsPlatform,