[Images] Update index.mdx (#21045)

Per https://developers.cloudflare.com/images/transform-images/#supported-formats-and-limitations

Author: nenizera

src/content/docs/images/upload-images/index.mdx

@@ -13,7 +13,7 @@ Cloudflare Images allows developers to upload images using different methods, fo
 You can upload the following image formats to Cloudflare Images:
 
 * PNG
-* GIF
+* GIF (including animations)
 * JPEG
 * WebP (Cloudflare Images also supports uploading animated WebP files)
 * SVG