Add developer quickstart sample

Author: justinyoo

docs/global.json

@@ -0,0 +1,5 @@
+{
+  "sdk": {
+    "version": "10.*-rc*"
+  }
+}

docs/overview/developer_quickstart.cs

@@ -0,0 +1,14 @@
+// SAMPLE: Generate text from a simple prompt
+// 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());