AI Gateway - Alternative way to handle automated batch completion events

## Description

Add documentation for an additional way of handling batch completion event, where a file is automatically retrieved after a batch job finishes. AI Gateway currently treats batching and the files API separately, so users need guidance on how to implement this pattern.

Context: https://konghq.atlassian.net/browse/AG-428

Document expected flow:

* User sends a batch request with a header or query flag indicating:
* Timer lifetime (timeout)
* Base URL where the batch result file can be fetched
* Gateway (or external script) polls the batch status until completion
* When batch completes, retrieve the file content from the provided base URL
* Users should enable `log_statistics` on the files route so metrics populate after file retrieval

## Definition of done

* Add a how-to section describing the batch polling + file fetch pattern in the alternative flow
* Include a diagram overview of the flow
* Add a section in AI Proxy (Advanced) docs about it

## Additional information

Person of contact: @oowl
**Size:** M