.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- Fixed file paths in samples section and removed outdated Azure integration reference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>

---------

Co-authored-by: Mark Wallace <127216156+markwallace-microsoft@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

dotnet/nuget/NUGET.md

@@ -1,10 +1,21 @@
 # About Microsoft Agent Framework
 
-To be filled in.
+Microsoft Agent Framework is a comprehensive .NET library for building, orchestrating, and deploying AI agents and multi-agent workflows. The framework provides everything from simple chat agents to complex multi-agent systems with graph-based orchestration capabilities.
+
+## Key Features
+
+- **Multi-Agent Orchestration**: Coordinate multiple agents using sequential, concurrent, group chat, and handoff patterns
+- **Graph-based Workflows**: Connect agents and deterministic functions with streaming, checkpointing, and human-in-the-loop capabilities
+- **Multiple Provider Support**: Seamlessly integrate with various LLM providers with more being added continuously
+- **Extensible Middleware**: Flexible request/response processing with custom pipelines and exception handling
+- **Built-in Observability**: OpenTelemetry integration for distributed tracing, monitoring, and debugging
+- **Cross-Platform**: Compatible with .NET 8.0, .NET Standard 2.0, and .NET Framework for broad deployment options
+
+Whether you're building simple AI assistants or complex multi-agent systems, Microsoft Agent Framework provides the tools and abstractions needed to create robust, scalable AI applications in .NET.
 
 # Getting Started ⚡
 
-- Learn more at the [documentation site](https://aka.ms/TBD).
-- Join the [Discord community](https://aka.ms/TBD).
-- Follow the team on [Semantic Kernel blog](https://aka.ms/TBD).
+- Learn more at the [documentation site](https://learn.microsoft.com/agent-framework/overview/agent-framework-overview).
+- Join the [Discord community](https://discord.gg/b5zjErwbQM).
+- Follow the team on [Semantic Kernel blog](https://devblogs.microsoft.com/semantic-kernel/).
 - Check out the [GitHub repository](https://github.com/microsoft/agent-framework) for the latest updates.