feat(onboarding): .NET snippet (#8307)

## About the changes
Quick-start for .NET

frontend/src/component/onboarding/dialog/snippets/dotnet.md

@@ -8,13 +8,47 @@ dotnet add package Newtonsoft.Json
 2\. Initialize Unleash
 ```csharp
 using Unleash;
+using Unleash.ClientFactory;
+
+public class Program
+{
+    public static async Task Main()
+    {
+        var settings = new UnleashSettings()
+        {
+            AppName = "unleash-onboarding-dotnet",
+            UnleashApi = new Uri("<YOUR_API_URL>"),
+            SendMetricsInterval = TimeSpan.FromSeconds(5),
+            CustomHttpHeaders = new Dictionary<string, string>()
+            {
+                {"Authorization","<YOUR_API_TOKEN>"}
+            }
+        };
+
+        var unleash =  new DefaultUnleash(settings);
+
+        while (true) {
+            Console.WriteLine($"Flag is enabled: {unleash.IsEnabled("<YOUR_FLAG>")}");
+            await Task.Delay(1000);
+        }
+    }
+}
+
+```
+
+---
+```csharp
 var settings = new UnleashSettings()
 {
     AppName = "unleash-onboarding-dotnet",
     UnleashApi = new Uri("<YOUR_API_URL>"),
     CustomHttpHeaders = new Dictionary<string, string>()
     {
-      {"Authorization","<YOUR_API_TOKEN>" }
+        {"Authorization",Environment.GetEnvironmentVariable("UNLEASH_API_KEY")}
     }
 };
 ```
+
+---
+- [SDK repository with documentation](https://github.com/Unleash/unleash-client-dotnet)
+- [.NET/C# SDK example with CodeSandbox](https://github.com/Unleash/unleash-sdk-examples/tree/main/Csharp)