fix(sample): dotnet sample (#2930)

Author: bruno-garcia

src/includes/configuration/sample-rate/dotnet.mdx

@@ -0,0 +1,21 @@
+```csharp
+using Sentry;
+
+using (SentrySdk.Init(o =>
+{
+    o.SampleRate = 0.25; // Capture 25%
+})
+{
+    // app code here
+}
+```
+
+```fsharp
+open Sentry
+
+use __ = SentrySdk.Init(fun o ->
+    o.SampleRate <- 0.25  // Capture 25%
+    ())
+
+// app code here
+```