Closes #14436: Add indexes for all GenericForeignKey fields (#14463)

* Closes #14436: Add PostgreSQL indexes for all GenericForeignKeys

* Add note about GFK indexes to developer docs

netbox/tenancy/migrations/0013_gfk_indexes.py

@@ -0,0 +1,17 @@
+# Generated by Django 4.2.7 on 2023-12-07 16:09
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('tenancy', '0012_contactassignment_custom_fields'),
+    ]
+
+    operations = [
+        migrations.AddIndex(
+            model_name='contactassignment',
+            index=models.Index(fields=['content_type', 'object_id'], name='tenancy_con_content_693ff4_idx'),
+        ),
+    ]