docs: add deprecation notice for PipelinePromptTemplate (#31999)

**PR title**: 
add deprecation notice for PipelinePromptTemplate

**PR message**: 
In the API documentation, PipelinePromptTemplate is marked as
deprecated, but this is not mentioned in the docs.

I'm submitting this PR to add a deprecation notice to the docs.

**Tests**:
N/A (documentation only)

---------

Co-authored-by: Mason Daugherty <github@mdrxy.com>

docs/docs/how_to/prompts_composition.ipynb

@@ -178,6 +178,18 @@
     "## Using PipelinePrompt"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "8ccadbae",
+   "metadata": {},
+   "source": [
+    ":::warning Deprecated\n",
+    "\n",
+    "PipelinePromptTemplate is deprecated; for more information, please refer to [PipelinePromptTemplate](https://python.langchain.com/api_reference/core/prompts/langchain_core.prompts.pipeline.PipelinePromptTemplate.html).\n",
+    "\n",
+    ":::"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "0a5892f9-e4d8-4b7c-b6a5-4651539b9734",