docs: Fix `CustomLLM` and `Question_answering` docs (#9782)

### Description
- Update `CustomLLM._call`: Corrected the _call method in CustomLLM to
include **kwargs, ensuring consistency with parent class.
- Update `Question_answering`: To fix `Page not found` error
- https://python.langchain.com/docs/use_cases/code ->
https://python.langchain.com/docs/use_cases/code_understanding

### Issue
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### Dependencies
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### Tag maintainer
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docs/extras/modules/model_io/models/llms/custom_llm.ipynb

@@ -52,6 +52,7 @@
     "        prompt: str,\n",
     "        stop: Optional[List[str]] = None,\n",
     "        run_manager: Optional[CallbackManagerForLLMRun] = None,\n",
+    "        **kwargs: Any,\n",
     "    ) -> str:\n",
     "        if stop is not None:\n",
     "            raise ValueError(\"stop kwargs are not permitted.\")\n",

docs/extras/use_cases/question_answering/question_answering.ipynb

@@ -14,7 +14,7 @@
     "\n",
     "In this walkthrough we'll go over how to build a question-answering over documents application using LLMs. Two very related use cases which we cover elsewhere are:\n",
     "- [QA over structured data](/docs/use_cases/sql) (e.g., SQL)\n",
-    "- [QA over code](/docs/use_cases/code) (e.g., Python)\n",
+    "- [QA over code](/docs/use_cases/code_understanding) (e.g., Python)\n",
     "\n",
     "![intro.png](/img/qa_intro.png)\n",
     "\n",