PDF Export URL pattern
### Describe the Bug
The web app calls `/REGION/LANG/wp-json/ig-mpdf/v1/pdf` to export a PDF which returns a 404. Our API currently uses `REGION/LANG/pdf`.

The normal mapping does not work, as we

### Steps to Reproduce

```shell
curl 'https://malte-test.tuerantuer.org/joerdenstorf/de/wp-json/ig-mpdf/v1/pdf'
```

### Expected Behavior
Map old URL pattern to new endpoint.


### Actual Behavior
404
diff --git a/integreat_cms/api/urls.py b/integreat_cms/api/urls.py
index 0244538a5..6f4a30225 100644
--- a/integreat_cms/api/urls.py
+++ b/integreat_cms/api/urls.py
@@ -136,6 +136,11 @@ urlpatterns = [
                     "<slug:language_slug>/wp-json/extensions/v3/",
                     include(content_api_urlpatterns),
                 ),
+                path(
+                    "<slug:language_slug>/wp-json/ig-mpdf/v1/pdf/",
+                    pdf_export,
+                    name="pdf_export",
+                ),
             ]
         ),
     ),
['integreat_cms/api/urls.py']