feat: Add UnstructuredRSTLoader (#6594)

### Summary

Adds an `UnstructuredRSTLoader` for loading
[reStructuredText](https://en.wikipedia.org/wiki/ReStructuredText) file.

### Testing

```python
from langchain.document_loaders import UnstructuredRSTLoader

loader = UnstructuredRSTLoader(
    file_path="example_data/README.rst", mode="elements"
)
docs = loader.load()
print(docs[0])
```

### Reviewers

- @hwchase17 
- @rlancemartin 
- @eyurtsev

docs/extras/modules/data_connection/document_loaders/integrations/rst.ipynb

@@ -0,0 +1,88 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# RST\n",
+    "\n",
+    ">A [reStructured Text (RST)](https://en.wikipedia.org/wiki/ReStructuredText) file is a file format for textual data used primarily in the Python programming language community for technical documentation."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## `UnstructuredRSTLoader`\n",
+    "\n",
+    "You can load data from RST files with `UnstructuredRSTLoader` using the following workflow."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.document_loaders import UnstructuredRSTLoader"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "loader = UnstructuredRSTLoader(\n",
+    "    file_path=\"example_data/README.rst\", mode=\"elements\"\n",
+    ")\n",
+    "docs = loader.load()"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "page_content='Example Docs' metadata={'source': 'example_data/README.rst', 'filename': 'README.rst', 'file_directory': 'example_data', 'filetype': 'text/x-rst', 'page_number': 1, 'category': 'Title'}\n"
+     ]
+    }
+   ],
+   "source": [
+    "print(docs[0])"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.11.3"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 4
+}