Upgrade NuGet packages to .NET 9.0.9 and create comprehensive documentation

[Copilot]

This PR addresses three key improvements to the xunit-dependency-injection library:

🔧 NuGet Package Upgrades

Updated all packages to the latest .NET 9.0 versions while maintaining compatibility:

• Azure Pipelines: Updated both pipeline YAML files to use .NET SDK 9.0.305
• Microsoft.Extensions packages: Upgraded from 9.0.8 → 9.0.9 (latest stable .NET 9.0 releases)
• xUnit packages: Upgraded from 3.0.1 → 3.1.0 and test runner from 3.1.4 → 3.1.5

All 43 example tests continue to pass, confirming the upgrades maintain full backward compatibility.

📖 New Examples.md Documentation

Created a comprehensive 19KB Examples.md file that serves as a complete developer guide:

• All injection patterns: Traditional fixture-based, property injection with [Inject] attributes, and factory patterns
• Advanced scenarios: Keyed services, configuration integration, user secrets, service lifetimes
• Working code samples: Every example is taken from the actual test suite and verified to work
• Migration guidance: Step-by-step instructions for adopting new features

📚 Enhanced README.md

Significantly expanded the README to better serve new .NET developers:

Before: Basic installation and minimal examples
After: Complete getting-started guide with:

• Prerequisites and development environment setup
• Multiple installation methods (Package Manager, .NET CLI, PackageReference)
• Quick-start example that gets developers running immediately
• Complete list of required dependencies with exact version numbers
• Troubleshooting section for common issues
• Prominent link to the new Examples.md documentation

🎯 Impact

These changes transform the developer experience from basic documentation to comprehensive guidance that enables both newcomers and experienced developers to effectively leverage dependency injection in their xUnit tests. The package upgrades ensure the library stays current with the latest .NET 9.0 ecosystem improvements.

The library now provides three distinct approaches for dependency injection, each with clear documentation and working examples, making it accessible to teams with different coding preferences and migration requirements.

Original prompt

Take care of the following tasks:

1. Create a GitHub issue to upgrade the applicable NuGet packages in .NET 9.0. Make sure you are not downgrading anything and you are not using .NET 10. Make sure the .NET SDK's version in the build pipelines' YAML files are ver "9.0.305".

2. Create another GitHub issue for producing a new markdown document to elaborate the examples in the source code. Call it Examples.md file.

3. Create the 3rd GitHub issue to expand the documentation of the library found in the README.md file. Elaborate the details of how to use the library including how to reference the NuGet package and all other further details. Make sure a new .NET developer wants to understand how to leverage this library from scratch with expansive and enough info. Also create a link to Examples.md file in the README.md file to lead the users to that document to understand the examples.

Create one and only one PR to fix the three issues above together and work on the PR.

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