feat: adds UnstructuredURLLoader for loading data from urls (#979)

### Summary

Adds a `UnstructuredURLLoader` that supports loading data from a list of
URLs.


### Testing

```python
from langchain.document_loaders import UnstructuredURLLoader

urls = [
    "https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-february-8-2023",
    "https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-february-9-2023"
]
loader = UnstructuredURLLoader(urls=urls)
raw_documents = loader.load()
```

docs/modules/document_loaders/examples/url.ipynb

@@ -0,0 +1,78 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "2dfc4698",
+   "metadata": {},
+   "source": [
+    "# URL\n",
+    "\n",
+    "This covers how to load HTML documents from a list of URLs into a document format that we can use downstream."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "16c3699e",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    " from langchain.document_loaders import UnstructuredURLLoader"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "id": "836fbac1",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "urls = [\n",
+    "    \"https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-february-8-2023\",\n",
+    "    \"https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-february-9-2023\"\n",
+    "]\n"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "00f46fda",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "loader = UnstructuredURLLoader(urls=urls)"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "id": "b68a26b3",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "data = loader.load()"
+   ]
+  }
+ ],
+ "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": "3.8.13"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}