Add banner for c# docs (#855)

Author: jordan-smith721

snooty.toml

@@ -15,3 +15,13 @@ csharp-docs-versioned = "https://mongodb.github.io/mongo-csharp-driver/{+csharp-
 
 java-version = "4.8"
 java-api = "https://mongodb.github.io/mongo-java-driver/{+java-version+}"
+
+[[banners]]
+targets = ["csharp.txt"]
+
+variant = "tip"
+value = """ \
+    A new version of the .Net/C# driver documentation is now live. To learn more about using \
+    the driver in your application, see the `official MongoDB C# driver documentation \
+    <https://www.mongodb.com/docs/drivers/csharp/current/>`__.
+    """