Create simpleprompt.cs

Example on tying openai-dotnet docs to openai.com docs folder structure

Author: achandmsft

docs/guides/text/chat/simpleprompt.cs

@@ -0,0 +1,12 @@
+// Generate text from a simple prompt using Chat Completions API (https://platform.openai.com/docs/guides/text?api-mode=chat)
+// This file can be run as an app using .NET 10 Preview 4 or later using the following steps 
+//  1. Set OPENAI_KEY environment variable to your OpenAI API key, never hardcode it.  
+//  2. Run this command: type chat_simpleprompt.cs
+//  3. Run this commnd: dotnet run chat_simpleprompt.cs 
+#:package OpenAI@2.*
+#:property PublishAot=false
+using OpenAI.Chat;
+string key = Environment.GetEnvironmentVariable("OPENAI_KEY")!;
+ChatClient client = new (model: "gpt-4.1", apiKey: apiKey); 
+ChatCompletion acompletion = client.CompleteChat("list all noble gases"); 
+Console.WriteLine(acompletion.Content[0].Text);