Add MVVM Toolkit installation instructions to Part 2 documentation

[Copilot]

Learners not starting from the workshop's starter project lack guidance on installing the required CommunityToolkit.Mvvm package.

Changes

Added installation note to Part 2 MVVM documentation in all language versions (English, Chinese Simplified, Chinese Traditional):

• Installation methods via Visual Studio GUI, .NET CLI, and Package Manager Console
• Link to official documentation
• Positioned at start of "Simplifying MVVM with .NET Community Toolkit" section, immediately before first usage

Context

The workshop includes CommunityToolkit.Mvvm globally via Directory.Build.targets, making it transparent to users following the provided projects. External learners applying workshop concepts to their own projects need explicit installation instructions.

> **Note for those not starting from the starter project:** If you're working on your own project or didn't start from Part 1, you'll need to install the CommunityToolkit.Mvvm NuGet package. You can do this via:
> - **Visual Studio**: Right-click on the project → Manage NuGet Packages → Search for "CommunityToolkit.Mvvm" → Install
> - **.NET CLI**: Run `dotnet add package CommunityToolkit.Mvvm`
> - **Package Manager Console**: Run `Install-Package CommunityToolkit.Mvvm`

Original prompt

Adding a quick note or link to the MVVM Toolkit installation would help new
learners avoid confusion if they aren't starting from the starter project

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