updater: import json at module level in doc_generator

_read_update_history is a staticmethod and used json without an import
(the only one was local to _run_incremental_update), so every --update
that reached the metadata rewrite crashed with NameError.

Author: anhnh2002

codewiki/cli/adapters/doc_generator.py

@@ -6,6 +6,7 @@
 """
 
 import asyncio
+import json
 import logging
 import os
 import sys