Added Sample code for Stack widget (#26586)

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

@@ -2851,6 +2851,74 @@ class ListBody extends MultiChildRenderObjectWidget {
 /// [CustomMultiChildLayout] instead. In particular, when using a [Stack] you
 /// can't position children relative to their size or the stack's own size.
 ///
+/// {@tool sample}
+///
+/// Using a [Stack] you can position widgets over one another.
+///
+/// ```dart
+/// Stack(
+///   children: <Widget>[
+///     Container(
+///       width: 100,
+///       height: 100,
+///       color: Colors.red,
+///     ),
+///     Container(
+///       width: 90,
+///       height: 90,
+///       color: Colors.green,
+///     ),
+///     Container(
+///       width: 80,
+///       height: 80,
+///       color: Colors.blue,
+///     ),
+///   ],
+/// )
+/// ```
+/// {@end-tool}
+///
+/// {@tool sample}
+///
+/// This example shows how [Stack] can be used to enhance text visibility
+/// by adding gradient backdrops.
+///
+/// ```dart
+/// SizedBox(
+///   width: 250,
+///   height: 250,
+///   child: Stack(
+///     children: <Widget>[
+///       Container(
+///         width: 250,
+///         height: 250,
+///         color: Colors.white,
+///       ),
+///       Container(
+///         padding: EdgeInsets.all(5.0),
+///         alignment: Alignment.bottomCenter,
+///         decoration: BoxDecoration(
+///           gradient: LinearGradient(
+///             begin: Alignment.topCenter,
+///             end: Alignment.bottomCenter,
+///             colors: <Color>[
+///               Colors.black.withAlpha(0),
+///               Colors.black12,
+///               Colors.black45
+///             ],
+///           ),
+///         ),
+///         child: Text(
+///           "Foreground Text",
+///           style: TextStyle(color: Colors.white, fontSize: 20.0),
+///         ),
+///       ),
+///     ],
+///   ),
+/// )
+/// ```
+/// {@end-tool}
+///
 /// See also:
 ///
 ///  * [Align], which sizes itself based on its child's size and positions