docs: overriding the default admin site (#1082)

Author: ciaransanders

docs/sites/index.md

@@ -1,7 +1,7 @@
 ---
 title: Custom sites
 order: 11
-description: Custom admin sites for Unfold.
+description: Create and customize admin sites in Django Unfold, including overriding the default admin site and registering models with custom admin sites.
 ---
 
 # Custom sites
@@ -10,6 +10,7 @@ In order to create a custom admin site, Unfold provides the `unfold.sites.Unfold
 
 ```python
 # sites.py
+
 from django.contrib import admin
 from unfold.sites import UnfoldAdminSite
 
@@ -26,6 +27,7 @@ custom_admin_site = CustomAdminSite(name="custom_admin_site")
 from django.urls import path
 from .sites import custom_admin_site
 
+
 urlpatterns = [
     # other URL patterns
     path("admin/", custom_admin_site.urls),
@@ -38,9 +40,35 @@ urlpatterns = [
 from django.contrib.auth.models import User
 from unfold.admin import ModelAdmin
 
+
 @admin.register(User, site=custom_admin_site)
 class UserAdmin(ModelAdmin):
     model = User
 ```
 
-Note: If you use the default `django.contrib.admin.AdminSite` you will receive a `NoReverseMatch` error because the default admin site does not contain all URL patterns required by Unfold.
+**Note**: If you use the default `django.contrib.admin.AdminSite` you will receive a `NoReverseMatch` error because the default admin site does not contain all URL patterns required by Unfold.
+
+## Overriding the default admin site
+
+If you want to override the default admin site by setting the `default_site` attribute of a custom `django.contrib.admin.apps.AdminConfig` class, you must install Unfold using `unfold.apps.AppConfig` instead of just `unfold` in `INSTALLED_APPS`.
+
+```python
+# settings.py
+
+INSTALLED_APPS = [
+    "unfold.apps.BasicAppConfig", # App config not overriding `django.contrib.admin.site`
+    # some other apps
+    "django.contrib.admin",
+    "your_app",
+]
+```
+
+```python
+# apps.py
+
+from django.contrib.admin.apps import AdminConfig
+
+
+class MyAdminConfig(AdminConfig):
+    default_site = "myproject.sites.CustomAdminSite"
+```