Dev Portal: Explain limitations of multiple auth plugins and ways around it

## Jobs to be done (optional)
Enabling an auth plugin on a Gateway and then configuring an auth strategy for a Dev Portal leads to complications because the credential can't pass both auth plugins. We do document multi-auth, but this isn't explicitly called out in Dev Portal docs.

## Definition of done
- Pull content from https://developer.konghq.com/gateway/authentication/#using-multiple-authentication-methods and explain the limitation and how to work around it with anonymous consumers and ACE.

## Information
Slack: https://kongstrong.slack.com/archives/CAW1MAFLZ/p1781642442075089?thread_ts=1781635187.195719&cid=CAW1MAFLZ

## Due date (optional)

## Size

Start with the Dev Portal authentication documentation and the linked Gateway page on using multiple authentication methods. Add an explanation of why credentials cannot pass both plugins, including the anonymous consumers and ACE workarounds, and verify that the new content matches the linked guidance.

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