Metering & Billing: Updates to generic metering content

## Definition of done
- Add a new generic metering section after https://developer.konghq.com/metering-and-billing/metering/#create-a-meter and Event Ingestion should move to an h3 under this heading
- Revise https://developer.konghq.com/metering-and-billing/metering/#aggregation-types (Peter: "They can only pick aggregation type for generic metering. So that can potentially move down too. APi GW requests is always COUNT and AI GW tokens is always SUM. But maybe worth explain top.")

## Information
DRP: Peter M.
Slack: https://kongstrong.slack.com/archives/C09BAEXTTM4/p1768231492799419

## Size
M

Start at the linked metering page and locate the “Create a meter,” “Event Ingestion,” and “Aggregation types” sections. Restructure the headings and revise the aggregation explanation to match the stated generic metering, API Gateway request, and AI Gateway token behavior; done means the page reflects all listed definition-of-done points.

Written by the indexing model from the issue text.