Add missing import django models (#8301)

Django models need to be imported

CONTRIBUTORS.rst

@@ -585,6 +585,7 @@ Contributors
 * Shwet Khatri
 * Abdulmajeed Isa
 * Sandil Ranasinghe
+* Caio Jhonny
 
 Translators
 ===========

docs/advanced_topics/documents/custom_document_model.md

@@ -14,6 +14,8 @@ Here's an example:
 
 ```python
 # models.py
+from django.db import models
+
 from wagtail.documents.models import Document, AbstractDocument
 
 class CustomDocument(AbstractDocument):