add `delete_all_documents()` operation to `AstraDocumentStore`

The operation should delete all the documents in the document store.

There seems to exist already in the API [a native operation](https://docs.datastax.com/en/astra-db-serverless/api-reference/document-methods/delete-many.html#:~:text=To%20delete%20all%20documents%2C%20use,in%20the%20collection%20was%20deleted.) to achieve this.