[document_page] page_id should be readonly.

Improve active field

document_page/models/document_page.py

@@ -21,6 +21,8 @@ class DocumentPage(models.Model):
         default="content"
     )
 
+    active = fields.Boolean(default=True)
+
     parent_id = fields.Many2one(
         'document.page',
         'Category',