docs: fix pdf docs hierarchy and formatting (#4593)

# Fix pdf loader docs page


![image](https://github.com/hwchase17/langchain/assets/707699/4a11f379-00ed-4f7a-9870-71f74e0cadc6)

Using h1's messes with hierarchy, this fixes that, and moves the
PyPDFium2 loader out of the middle of PDFMiner docs

docs/modules/indexes/document_loaders/examples/pdf.ipynb

@@ -337,75 +337,73 @@
   },
   {
    "cell_type": "markdown",
-   "id": "05187b33",
+   "id": "96351714",
-   "metadata": {},
-   "source": []
-  },
-  {
-   "cell_type": "markdown",
-   "id": "21998d18",
    "metadata": {},
    "source": [
-    "## Using PDFMiner"
+    "## Using PyPDFium2"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 7,
+   "execution_count": 1,
-   "id": "2f0cc9ff",
+   "id": "003fcc1d",
    "metadata": {},
    "outputs": [],
    "source": [
-    "from langchain.document_loaders import PDFMinerLoader"
+    "from langchain.document_loaders import PyPDFium2Loader"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 8,
+   "execution_count": 3,
-   "id": "42b531e8",
+   "id": "46766e29",
    "metadata": {},
    "outputs": [],
    "source": [
-    "loader = PDFMinerLoader(\"example_data/layout-parser-paper.pdf\")"
+    "loader = PyPDFium2Loader(\"example_data/layout-parser-paper.pdf\")"
    ]
   },
   {
    "cell_type": "code",
    "execution_count": 9,
-   "id": "483720b5",
-   "metadata": {},
    "outputs": [],
    "source": [
     "data = loader.load()"
-   ]
+   ],
+   "metadata": {
+    "collapsed": false
+   }
   },
   {
    "cell_type": "markdown",
-   "id": "96351714",
-   "metadata": {},
    "source": [
-    "# Using PyPDFium2"
+    "## Using PDFMiner"
-   ]
+   ],
+   "metadata": {
+    "collapsed": false
+   }
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": 7,
-   "id": "003fcc1d",
-   "metadata": {},
    "outputs": [],
    "source": [
-    "from langchain.document_loaders import PyPDFium2Loader"
+    "from langchain.document_loaders import PDFMinerLoader"
-   ]
+   ],
+   "metadata": {
+    "collapsed": false
+   }
   },
   {
    "cell_type": "code",
-   "execution_count": 3,
+   "execution_count": 8,
-   "id": "46766e29",
-   "metadata": {},
    "outputs": [],
    "source": [
-    "loader = PyPDFium2Loader(\"example_data/layout-parser-paper.pdf\")"
+    "loader = PDFMinerLoader(\"example_data/layout-parser-paper.pdf\")"
-   ]
+   ],
+   "metadata": {
+    "collapsed": false
+   }
   },
   {
    "cell_type": "code",
@@ -422,7 +420,7 @@
    "id": "c90a5fe8",
    "metadata": {},
    "source": [
-    "## Using PDFMiner to generate HTML text"
+    "### Using PDFMiner to generate HTML text"
    ]
   },
   {