fix: revert document content field rename (#6955)

Signed-off-by: Alexander Onnikov <Alexander.Onnikov@xored.com>

models/server-document/src/index.ts

@@ -5,9 +5,9 @@
 
 import { type Builder } from '@hcengineering/model'
 
-import core from '@hcengineering/core'
+import core, { type Class, type Doc } from '@hcengineering/core'
 import document from '@hcengineering/document'
-import serverCore from '@hcengineering/server-core'
+import serverCore, { type ObjectDDParticipant } from '@hcengineering/server-core'
 import serverDocument from '@hcengineering/server-document'
 import serverNotification from '@hcengineering/server-notification'
 import serverView from '@hcengineering/server-view'
@@ -36,4 +36,13 @@ export function createModel (builder: Builder): void {
       title: 'title'
     }
   })
+
+  builder.mixin<Class<Doc>, ObjectDDParticipant>(
+    document.class.Document,
+    core.class.Class,
+    serverCore.mixin.ObjectDDParticipant,
+    {
+      collectDocs: serverDocument.function.FindChildDocuments
+    }
+  )
 }