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

packages/flutter_tools/lib/src/linux/build_linux.dart

@@ -38,7 +38,7 @@ Future<void> buildLinux(
   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.');
   }
 

packages/flutter_tools/lib/src/macos/build_macos.dart

@@ -31,7 +31,7 @@ Future<void> buildMacOS({
 }) 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.');
   }
 

packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -32,7 +32,7 @@ Future<void> buildWindows(WindowsProject windowsProject, BuildInfo buildInfo, {
   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 @@ Future<void> buildWindowsUwp(WindowsUwpProject windowsProject, BuildInfo buildIn
   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.',
     );
   }

packages/flutter_tools/test/commands.shard/hermetic/build_linux_test.dart

@@ -114,7 +114,7 @@ void main() {
     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,

packages/flutter_tools/test/commands.shard/hermetic/build_macos_test.dart

@@ -131,7 +131,7 @@ void main() {
     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,

packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart

@@ -159,7 +159,7 @@ void main() {
     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 @@ C:\foo\windows\runner\main.cpp(17,1): error C2065: 'Baz': undeclared identifier
     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,