docs: reference ExperimentalMarkdownTextSplitter (#28768)

Continuing https://github.com/langchain-ai/langchain/pull/27832

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Co-authored-by: promptless[bot] <179508745+promptless[bot]@users.noreply.github.com>
Co-authored-by: Frances Liu <francestfls@gmail.com>

docs/docs/how_to/markdown_header_metadata_splitter.ipynb

@@ -162,6 +162,18 @@
     "md_header_splits"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "fb3f834a",
+   "metadata": {},
+   "source": [
+    ":::note\n",
+    "\n",
+    "The default `MarkdownHeaderTextSplitter` strips white spaces and new lines. To preserve the original formatting of your Markdown documents, check out [ExperimentalMarkdownSyntaxTextSplitter](https://python.langchain.com/api_reference/text_splitters/markdown/langchain_text_splitters.markdown.ExperimentalMarkdownSyntaxTextSplitter.html).\n",
+    "\n",
+    ":::"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "aa67e0cc-d721-4536-9c7a-9fa3a7a69cbe",