[docs][FWW] DropdownButton, ScaffoldMessenger, and StatefulBuilder links (#100316)

* added dropdown and scaffoldmessenger FWW links

* added StatefulBuilder wotw link

* added Autocomplete FWW link

* removed accidental auto-formatting

packages/flutter/lib/src/material/autocomplete.dart

@@ -26,6 +26,8 @@ import 'theme.dart';
 /// ** See code in examples/api/lib/material/autocomplete/autocomplete.1.dart **
 /// {@end-tool}
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=-Nny8kzW380}
+///
 /// See also:
 ///
 ///  * [RawAutocomplete], which is what Autocomplete is built upon, and which

packages/flutter/lib/src/material/dropdown.dart

@@ -826,6 +826,8 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 ///
 /// Requires one of its ancestors to be a [Material] widget.
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=ZzQ_PWrFihg}
+///
 /// See also:
 ///
 ///  * [DropdownButtonFormField], which integrates with the [Form] widget.

packages/flutter/lib/src/material/scaffold.dart

@@ -84,6 +84,8 @@ enum _ScaffoldSlot {
 /// ** See code in examples/api/lib/material/scaffold/scaffold_messenger.0.dart **
 /// {@end-tool}
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=lytQi-slT5Y}
+///
 /// See also:
 ///
 ///  * [SnackBar], which is a temporary notification typically shown near the

packages/flutter/lib/src/widgets/basic.dart

@@ -7448,6 +7448,8 @@ typedef StatefulWidgetBuilder = Widget Function(BuildContext context, StateSette
 ///
 /// {@tool snippet}
 ///
+/// {@youtube 560 315 https://www.youtube.com/watch?v=syvT63CosNE}
+///
 /// This example shows using an inline StatefulBuilder that rebuilds and that
 /// also has state.
 ///