Add new interface (#504)

Author: AlexGaillard

content/guides/01.data-model/3.interfaces.md

@@ -555,7 +555,20 @@ Dropdown to select an item from an existing collection.
 | Configuration    | Options                                                                                                                      |
 | ---------------- | ---------------------------------------------------------------------------------------------------------------------------- |
 | Types            | `JSON`                                                                                                                       |
-| Collection       | Which collection should items be selected from.                                                                              |
+| Collection       | Which collection should the item be selected from.                                                                           |
+| Display Template | Display templates are used to represent an item in relationship fields - for example to show the value of the `Title` field. |
+| Filter           | Filter the list of items a user can select.                                                                                  |
+
+### Collection Item Dropdown (Multiple)
+
+![A collection multiple item field](/img/b2cfb9dc-d9f2-4edf-a014-1ec1e8e7102e.webp)
+
+Dropdown to select multiple items from an existing collection.
+
+| Configuration    | Options                                                                                                                      |
+| ---------------- | ---------------------------------------------------------------------------------------------------------------------------- |
+| Types            | `JSON`                                                                                                                       |
+| Collection       | Which collection should the items be selected from.                                                                          |
 | Display Template | Display templates are used to represent an item in relationship fields - for example to show the value of the `Title` field. |
 | Filter           | Filter the list of items a user can select.                                                                                  |