feat(media): add external media library support, Uploadcare integration (#1602)

website/content/docs/widgets/image.md

@@ -10,10 +10,20 @@ The image widget allows editors to upload an image or select an existing one fro
 - **Data type:** file path string, based on `media_folder`/`public_folder` configuration
 - **Options:**
   - `default`: accepts a file path string; defaults to null
+  - `options`: an object for settings that are unique to the image widget
+    - `media_library`: media library settings to apply when a media library is opened by the
+      current widget
+      - `config`: a configuration object that will be passed directly to the media library being
+        used - available options are determined by the library
 - **Example:**
     ```yaml
     - label: "Featured Image"
       name: "thumbnail"
       widget: "image"
       default: "/uploads/chocolate-dogecoin.jpg"
+      options:
+        media_library:
+          config:
+            publicKey: "demopublickey"
+            multiple: true
     ```