Replace WPF sample with Avalonia sample in README

[Copilot]

The README's ProgressContainer usage example referenced WPF-specific XAML, while the demo project already uses Avalonia.

Changes

• Readme.md — Updated the ProgressContainer code sample:
  • Intro text: "WPF application" → "Avalonia application"
  • XAML namespace: http://schemas.microsoft.com/winfx/2006/xaml/presentation → https://github.com/avaloniaui
  • Removed WPF design-time namespace (xmlns:d) and d:DataContext attribute
  • Added x:DataType="MainViewModel" for Avalonia compiled bindings

<Window
    x:Class="MainWindow"
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:DataType="MainViewModel">
    ...
</Window>

Original prompt

Edit the readme by replacing the WPF sample with an Avalonia sample

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[Copilot]

Pull request overview

This PR updates the README documentation to replace a WPF-based XAML example with an Avalonia-based example, aligning the documentation with the actual demo project which uses Avalonia UI framework.

Changes:

• Updated descriptive text to reference "Avalonia application" instead of "WPF application"
• Replaced WPF XAML namespace with Avalonia namespace (https://github.com/avaloniaui)
• Removed WPF design-time attributes and replaced with Avalonia's x:DataType for compiled bindings support

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