.NET Samples - Create 03-workflows learning path step (#4102)

dotnet/samples/03-workflows/Visualization/README.md

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+# Workflow Visualization Sample
+
+This sample demonstrates how to visualize workflows using `ToMermaidString()` and `ToDotString()` extension methods. It uses a map-reduce workflow with fan-out/fan-in patterns as an example.
+
+## Running the Sample
+
+```bash
+dotnet run
+```
+
+## Output Formats
+
+The sample generates two visualization formats:
+
+### Mermaid
+Paste the output into any Mermaid-compatible viewer (GitHub, Mermaid Live Editor, etc.):
+
+![Mermaid Visualization](Resources/mermaid_render.png)
+
+### DOT (Graphviz)
+Render with Graphviz (requires `graphviz` to be installed):
+
+```bash
+dotnet run | tail -n +20 | dot -Tpng -o workflow.png
+```
+
+![Graphviz Visualization](Resources/graphviz_render.png)
+
+## Usage
+
+```csharp
+Workflow workflow = BuildWorkflow();
+
+// Generate Mermaid format
+string mermaid = workflow.ToMermaidString();
+
+// Generate DOT format
+string dotString = workflow.ToDotString();
+```