Clarify Arize AX integration guidance

[arizedatngo]

Summary

• Clarifies Arize AX as the full-featured observability and evaluation option for OpenRouter traces
• Notes the Phoenix path for teams using an open-source workflow
• Adds concise evaluation context for readers using captured traces to review model and agent behavior

Why

The existing pages already show how to send OpenRouter traces to Arize AX. This update improves product context by noting AX support for managed cloud and enterprise self-hosted deployments, while pointing local and self-hosted experimentation workflows to Phoenix.

Testing

• Docs-only change; not run.

[arizedatngo]

@kenrogers you merged this update originally; could you help route a publication regression to the docs sync source owner? Sync #307 / commit 20c24a7 later removed the AX/Phoenix context and both evaluation links from guides/community/arize.mdx and guides/features/broadcast/arize.mdx, and they remain absent publicly.

The intended current targets are:

• Agent evaluation: https://arize.com/guides/ai-agent-handbook/agent-evaluation/
• LLM evaluation: https://arize.com/resources/llm-evaluation/

I have prepared a two-page restoration with different SDK/Broadcast wording and a clear distinction between Arize AI (company), AX (enterprise product), and Phoenix (open source). The Broadcast copy explicitly describes AX configuration. Which upstream source should receive this so the next sync preserves it? The referenced web source repository is not accessible to this account.

Reviewable restoration draft: #490. It remains draft pending the upstream source change.