.NET: Nuget update (#987)
* Update README with enhanced highlights and streamlined getting started
- Added banner image for professional branding
- Updated highlights section to focus on key features: multi-agent orchestration, workflows, AF Labs, DevUI, Python/C# support, observability, multiple providers, and middleware
- Streamlined getting started section with direct links to comprehensive guides
- Fixed file paths in samples section and removed outdated Azure integration reference
- Improved overall structure and readability
* Update NuGet package documentation and add framework icon
- Added comprehensive high-level summary for Microsoft Agent Framework
- Included key features highlighting multi-agent orchestration, workflows, observability, and cross-platform support
- Added agent-framework-icon.png for NuGet package branding
* Update README.md
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
* Update dotnet/nuget/NUGET.md
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
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Co-authored-by: Mark Wallace <127216156+markwallace-microsoft@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
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