docs: add ImageGenerationExample (#457)

* ImageGenerationExample

* docs: slight example tweaks

---------

Co-authored-by: Tomer Aberbach <tomer@stainless.com>

Author: nuclear-bean

openai-java-example/src/main/java/com/openai/example/ImageGenerationExample.java

@@ -0,0 +1,30 @@
+package com.openai.example;
+
+import com.openai.client.OpenAIClient;
+import com.openai.client.okhttp.OpenAIOkHttpClient;
+import com.openai.models.images.ImageGenerateParams;
+import com.openai.models.images.ImageModel;
+import java.io.IOException;
+
+public final class ImageGenerationExample {
+    private ImageGenerationExample() {}
+
+    public static void main(String[] args) throws IOException {
+        // Configures using one of:
+        // - The `OPENAI_API_KEY` environment variable
+        // - The `OPENAI_BASE_URL` and `AZURE_OPENAI_KEY` environment variables
+        OpenAIClient client = OpenAIOkHttpClient.fromEnv();
+
+        ImageGenerateParams imageGenerateParams = ImageGenerateParams.builder()
+                .responseFormat(ImageGenerateParams.ResponseFormat.URL)
+                .prompt("Two cats playing ping-pong")
+                .model(ImageModel.DALL_E_2)
+                .size(ImageGenerateParams.Size._512X512)
+                .n(1)
+                .build();
+
+        client.images().generate(imageGenerateParams).data().orElseThrow().stream()
+                .flatMap(image -> image.url().stream())
+                .forEach(System.out::println);
+    }
+}