Discontinued: /_api/export

ArangoDB's HTTP Interface for Bulk Exports was previously discontinued in favour of AQL queries and streaming cursors. 

i.e this no longer works
```
POST /_api/export?colleciton=cities
``` 

We should update https://docs.python-arango.com/en/main/specs.html#arango.collection.Collection.export to notify users that this functionality has been discontinued.


