docs(og-image): add excludeRoot config + update preview images

docs/features/social images.md

@@ -27,6 +27,7 @@ generateSocialImages: {
   colorScheme: "lightMode", // what colors to use for generating image, same as theme colors from config, valid values are "darkMode" and "lightMode"
   width: 1200, // width to generate with (in pixels)
   height: 630, // height to generate with (in pixels)
+  excludeRoot: false, // wether to exclude "/" index path to be excluded from auto generated images (false = use auto, true = use default og image)
   imageStructure: defaultImage // import from `socialImage.tsx`, recommended to add your own one there as well
 }
 ```