docs: adds detail about maintaining aspect ratio

docs/upload/overview.mdx

@@ -66,6 +66,16 @@ const Media = {
         width: 768,
         height: 1024,
         crop: 'centre',
+      },
+      {
+        name: 'tablet',
+        width: 1024,
+        // By specifying `null` or leaving a height undefined,
+        // the image will be sized to a certain width,
+        // but it will retain its original aspect ratio
+        // and calculate a height automatically.
+        height: null,
+        crop: 'centre',
       }
     ],
     adminThumbnail: 'thumbnail',