Expand README with full solution overview and per-package documentation

[Copilot]

The README was a minimal placeholder with no framework requirements, installation instructions, feature descriptions, or contribution guidance.

Changes

• Badges — added MIT license badge and NuGet version badges for all 8 packages alongside the existing CI badge
• Package table — summary of all packages with target frameworks (netstandard2.0/2.1, net8.0, net9.0, net9.0-windows) and one-line descriptions
• Requirements — .NET SDK version links and .NET Standard compatibility note
• Installation — .NET CLI and Package Manager Console commands for every package
• Per-package sections — feature highlights and quick-start snippets for each package (Core, Data, CosmosDb, EntityFramework, DependencyInjection, MediatR, Web, WPF)
• Building from source — clone → build → test instructions
• Contributing — process steps with links to CODE_OF_CONDUCT.md and SECURITY.md
• License — MIT attribution footer

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.