FAQ for how to send managed data plane metrics to Datadog plugin

## Jobs to be done (optional)
Users want to know how to send metrics for managed data planes to the datadog plugin.

## Definition of done
- Add FAQ to the following pages that explains how managed data planes can send metrics to the Datadog plugin:
- https://developer.konghq.com/plugins/datadog/
- https://developer.konghq.com/how-to/collect-metrics-with-datadog-and-prometheus-plugin/

## Information
Slack for request/FAQ answer: https://kongstrong.slack.com/archives/C070FKK6GBT/p1762465919121259

## Size
small

Start with the Datadog plugin page and the metrics collection how-to named in the issue, then review the linked Slack thread for the requested FAQ answer. Update both pages with guidance for sending managed data plane metrics to the Datadog plugin; done means the explanation appears on both pages and matches the request.

Written by the indexing model from the issue text.