[DOC] Fix #29685: HuggingFaceEndpoint missing task argument in documentation (#29686)

## **Description**
This PR updates the LangChain documentation to address an issue where
the `HuggingFaceEndpoint` example **does not specify the required `task`
argument**. Without this argument, users on `huggingface_hub == 0.28.1`
encounter the following error:

```
ValueError: Task unknown has no recommended model. Please specify a model explicitly.
```

---

## **Issue**
Fixes #29685

---

## **Changes Made**
✅ **Updated `HuggingFaceEndpoint` documentation** to explicitly define
`task="text-generation"`:
```python
llm = HuggingFaceEndpoint(
    repo_id=GEN_MODEL_ID,
    huggingfacehub_api_token=HF_TOKEN,
    task="text-generation"  # Explicitly specify task
)
```

✅ **Added a deprecation warning note** and recommended using
`InferenceClient`:
```python
from huggingface_hub import InferenceClient
from langchain.llms.huggingface_hub import HuggingFaceHub

client = InferenceClient(model=GEN_MODEL_ID, token=HF_TOKEN)

llm = HuggingFaceHub(
    repo_id=GEN_MODEL_ID,
    huggingfacehub_api_token=HF_TOKEN,
    client=client,
)
```

---

## **Dependencies**
- No new dependencies introduced.
- Change only affects **documentation**.

---

## **Testing**
- ✅ Verified that adding `task="text-generation"` resolves the issue.
- ✅ Tested the alternative approach with `InferenceClient` in Google
Colab.

---

## **Twitter Handle (Optional)**
If this PR gets announced, a shout-out to **@AkmalJasmin** would be
great! 🚀

---

## **Reviewers**
📌 **@langchain-maintainers** Please review this PR. Let me know if
further changes are needed.

🚀 This fix improves **developer onboarding** and ensures the **LangChain
documentation remains up to date**! 🚀

docs/docs/integrations/document_loaders/docling.ipynb

@@ -443,6 +443,7 @@
     "llm = HuggingFaceEndpoint(\n",
     "    repo_id=GEN_MODEL_ID,\n",
     "    huggingfacehub_api_token=HF_TOKEN,\n",
+    "    task=\"text-generation\",\n",
     ")"
    ]
   },