Document group_name customization UI hint

Author: shivansh

fern/docs/img/customization-group_name-ui-hint.png

@@ -391,6 +391,9 @@ hints:
 * `is_sortable`: Whether the field is sortable. Requires `is_filterable` to be true.
 * `is_groupable`: Whether the field is groupable. Requires `is_filterable` to be true.
 * `order`: The order in which the field appears in the side-panel.
+* `group_name`: The group title under which field(s) appear in the side-panel. In the example below, the fields are grouped under groups titled _Group One_ and _Group Two_.
+
+  ![customization-group_name-ui-hint](../img/customization-group_name-ui-hint.png)
 
 <Callout intent="tip">
 `is_filterable` is not a UI hint but a top level field property.