Metering & Billing: Explain how to use filtering to find customers by key

## Jobs to be done (optional)
In v1 API you could call API with key a pattern we moved away and now we only allow calls by ID. For example:
GET /api/v1/customers/my-key vs. GET /v3/openmeter/customers/01KN24J4S9T7EHC6M7NEJE308Y. With v3 we recommend to use list APIs to lookup key->ID like GET /v3/openmeter/customers?filter[key][eq]=my-key. Looks like this confuses some users. Can we add a section to docs to give examples around lookup and how to use keys? Keys are read-only and allows users to use their own database ID to make integration easier. Every single entity in OpenMeter has a key and supports this pattern. We had [this doc](https://openmeter.io/docs/billing/customer/overview#bring-your-id) specifically for customer in OM docs but this behavior changed no and new pattern is universal across all entities.

## Definition of done
- Update https://developer.konghq.com/metering-and-billing/customer/ with this information and why you'd want to use it

## Information
DRP: Peter M.
Slack: https://kongstrong.slack.com/archives/C09BAEXTTM4/p1774967238157629

## Due date (optional)

## Size
M

Start with the metering and billing customer page named in the definition of done, then compare it with the linked OpenMeter customer documentation. Add examples explaining key-to-ID lookup through the list API, why keys help integrations, and that the pattern applies to all entities; the customer page should contain this guidance when finished.

Written by the indexing model from the issue text.