Add support for running the lab in GitHub Codespaces

[Copilot]

Overview

This PR adds comprehensive GitHub Codespaces support to the .NET AI Workshop, enabling users to run the entire lab in their browser without any local installation requirements. This addresses scenarios where users can't install Docker locally or encounter setup issues.

What's New

🚀 One-Click Launch

Users can now click a badge to launch a fully configured development environment:

The Codespace automatically includes:

• .NET 9.0 SDK with all required workloads
• Docker-in-Docker for running Qdrant vector database
• Azure CLI, Azure Developer CLI, and GitHub CLI
• Microsoft.Extensions.AI.Templates pre-installed
• VS Code extensions (GitHub Copilot, C# Dev Kit)
• Part 2 starting solution pre-restored and ready to use

📁 Devcontainer Configuration

Added root-level .devcontainer/ folder with:

• devcontainer.json: Configures the Codespace environment following .NET Aspire devcontainer best practices
• post-create.sh: Automated setup script that runs after Codespace creation
• README.md: Comprehensive documentation for Codespace users including secret management and troubleshooting

Port forwarding is pre-configured for the Aspire dashboard (port 18888 and range 15000-15004).

📖 Enhanced Documentation

Updated documentation throughout the workshop to guide Codespace users:

• Main README: Added quick start section highlighting Codespaces as the fastest way to begin
• Part 1 - Setup: Promoted Codespaces as the recommended option with clear benefits
• Part 2 - Project Creation: Added CLI instructions for Codespace users alongside Visual Studio instructions

Key additions include:

• How to set user secrets using dotnet user-secrets CLI
• How to run the application from the terminal
• How to manage GitHub tokens and Azure credentials in Codespaces
• Troubleshooting guidance for Codespace-specific scenarios

Key Benefits

✅ Zero local setup required - Start coding immediately in the browser
✅ Solves Docker installation issues - Perfect for users who can't install Docker locally
✅ Consistent environment - Everyone gets identical pre-configured setup
✅ Ideal for workshops - Participants can be productive in minutes
✅ Maintains flexibility - Existing local development workflows unchanged

Backward Compatibility

This PR maintains full backward compatibility:

• Original devcontainer configurations preserved in .github/.devcontainer/
• All existing local development workflows continue to work exactly as before
• No changes to any project code or build configurations

Testing

• ✅ All solutions build successfully (Part 2, Part 6, Part 7, Part 8)
• ✅ Markdown linting passes for all documentation
• ✅ Devcontainer JSON structure validated
• ✅ Follows .NET Aspire devcontainer best practices

Closes #[issue-number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add support for running the lab in GitHub Codespace</issue_title>
<issue_description>The lab should be able to run in GitHub Codespace. Use the documentation in https://learn.microsoft.com/en-us/dotnet/aspire/get-started/dev-containers. I believe we should have a starting solution folder that runs in Codespace and the users will be able to follow the steps in the lab for cases where they can't install Docker or have other setup issues.</issue_description>

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

Fixes #209

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