feat: Add custom pane example to NavigationView

[Copilot]

NavigationPane.customPane accepts a NavigationPaneWidget for fully custom pane layouts, but lacked any example or documentation showing how to use it.

Changes

• Example app: Added a "Custom pane" section to NavigationViewPage with a live demo and code snippet showing NavigationPaneWidget usage
• _CustomNavigationPane: New private class in the example demonstrating how to extend NavigationPaneWidget and compose data.content into a custom layout
• Changelog: Added entry under [next]

Usage

class CustomNavigationPane extends NavigationPaneWidget {
  @override
  Widget build(BuildContext context, NavigationPaneWidgetData data) {
    return Row(
      children: [
        Container(
          width: 200,
          color: Colors.red.withValues(alpha: 0.2),
          child: const Center(child: Text('Custom pane')),
        ),
        Expanded(child: data.content),
      ],
    );
  }
}

NavigationView(
  pane: NavigationPane(
    customPane: CustomNavigationPane(),
    items: [...],
  ),
)

NavigationPaneWidgetData provides content, titleBar, scrollController, paneKey, listKey, and pane for full control over the layout.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Improve developer experience for a custom pane in navigation view.</issue_title>
<issue_description>Implemented NavigationPane.customPane, which now gives you the ability to create custom panes for NavigationView</issue_description>

<agent_instructions>Suggest a more dev-friendly way to use custom panes and implement it. Add a custom pane to the example app. In the end, update the changelog.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@bdlukaa The implementation for a custom pane is very simple, but it could definitely get some love.

First, create a NavigationPaneWidget, similar to creating a stateless widget:

class CustomNavigationPane extends NavigationPaneWidget {
  CustomNavigationPane();

  @override
  Widget build(BuildContext context, NavigationPaneWidgetData data) {
    return Row(
      children: [
        Container(
          color: Colors.red,
          child: Text('This is a custom navigation pane'),
        ),
        Expanded(child: data.content),
      ],
    );
  }
}

Then, provide it in your navigation view tree:

NavigationView(
  pane: NavigationPane(
    customPane: CustomNavigationPane(),
    ...
  ),
  ...
),
```</body></comment_new>
</comments>

• Fixes Improve developer experience for a custom pane in navigation view. #1194

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[bdlukaa]

@copilot Address #1025 in this PR. Improve NavigationView documentation, showing off the various features it has, with code examples in the /example project.