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

tests/integration_tests/document_loaders/test_rst.py

@@ -0,0 +1,15 @@
+import os
+from pathlib import Path
+
+from langchain.document_loaders import UnstructuredRSTLoader
+
+EXAMPLE_DIRECTORY = file_path = Path(__file__).parent.parent / "examples"
+
+
+def test_unstructured_rst_loader() -> None:
+    """Test unstructured loader."""
+    file_path = os.path.join(EXAMPLE_DIRECTORY, "README.rst")
+    loader = UnstructuredRSTLoader(str(file_path))
+    docs = loader.load()
+
+    assert len(docs) == 1

tests/integration_tests/examples/README.rst

@@ -0,0 +1,28 @@
+Example Docs
+------------
+
+The sample docs directory contains the following files:
+
+-  ``example-10k.html`` - A 10-K SEC filing in HTML format
+-  ``layout-parser-paper.pdf`` - A PDF copy of the layout parser paper
+-  ``factbook.xml``/``factbook.xsl`` - Example XML/XLS files that you
+   can use to test stylesheets
+
+These documents can be used to test out the parsers in the library. In
+addition, here are instructions for pulling in some sample docs that are
+too big to store in the repo.
+
+XBRL 10-K
+^^^^^^^^^
+
+You can get an example 10-K in inline XBRL format using the following
+``curl``. Note, you need to have the user agent set in the header or the
+SEC site will reject your request.
+
+.. code:: bash
+
+   curl -O \
+     -A '${organization} ${email}'
+     https://www.sec.gov/Archives/edgar/data/311094/000117184321001344/0001171843-21-001344.txt
+
+You can parse this document using the HTML parser.