[Animations] add example to openCntainer

[AbdeMohlbi]

Fixes the todo : TODO(goderbauer): Add example animations and sample code.

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[AbdeMohlbi]

This change should not require a version bump since it is a documentation change mostly

[gemini-code-assist]

Code Review

This pull request adds a dartdoc example for the OpenContainer widget and updates the CHANGELOG.md to reflect this change. The example demonstrates how to use OpenContainer with openBuilder and closedBuilder. My review provides a suggestion to simplify the example code for clarity and adherence to Dart style conventions.

[gemini-code-assist]

For a clearer and more idiomatic example, I have a few suggestions:

• In closedBuilder, the GestureDetector is redundant because OpenContainer is tappable by default. It's simpler to rely on the default behavior. The action callback is useful for more complex scenarios, like when tappable is set to false.
• The action parameters in both openBuilder and closedBuilder are unused in this example. It's a good practice to rename them to _ to signify they are intentionally ignored.
• Per the Dart style guide, it's preferred to use single quotes for strings.

I've applied these changes in the suggestion.

/// OpenContainer(
///   transitionDuration: const Duration(milliseconds: 500),
///   transitionType: ContainerTransitionType.fadeThrough,
///   openBuilder: (context, _) {
///     return Scaffold(
///       appBar: AppBar(title: const Text('Details Page')),
///       body: const Center(
///         child: Text(
///           'This page opened with Container Transform animation',
///           style: TextStyle(fontSize: 18),
///           textAlign: TextAlign.center,
///         ),
///       ),
///     );
///   },
///   closedBuilder: (context, _) {
///     return Container(
///       width: 200,
///       height: 120,
///       alignment: Alignment.center,
///       decoration: BoxDecoration(
///         color: Colors.blue,
///         borderRadius: BorderRadius.circular(16),
///       ),
///       child: const Text(
///         'Open Details',
///         style: TextStyle(color: Colors.white, fontSize: 18),
///       ),
///     );
///   },
/// ),

References

1. DO use single quotes for strings. ^(link)

[AbdeMohlbi]

valid point.
not sure about the action with _.
valid point for single quotes.