Add code samples for the overview page (#685)

* Add developer quickstart sample

* Update header of the sample code

* Update the folder structure

Author: justinyoo

docs/overview/responses/developer_quickstart.cs

@@ -0,0 +1,15 @@
+// SAMPLE: Generate text from a simple prompt through Responses API
+// PAGE: https://platform.openai.com/docs/overview
+// GUIDANCE: Instructions to run this code: https://aka.ms/oai/net/start
+#pragma warning disable OPENAI001
+
+#:package OpenAI@2.*
+#:property PublishAot=false
+
+using OpenAI.Responses;
+
+string key = Environment.GetEnvironmentVariable("OPENAI_API_KEY")!;
+OpenAIResponseClient client = new(model: "gpt-5", apiKey: key);
+OpenAIResponse response = client.CreateResponse("Write a short bedtime story about a unicorn.");
+
+Console.WriteLine(response.GetOutputText());