docs: Clarify Arize tracing options

[arizedatngo]

Summary

Clarifies the Arize interoperability example in the Agent Spec tracing docs:

• distinguishes Arize Phoenix as the open-source/local or self-hosted tracing path
• points production users to the Arize AX Agent Spec tracing docs
• adds canonical Arize agent evaluation and LLM evaluation resources
• fixes the wording in the sample comment from Phoenix Arize server to Arize Phoenix

This is a docs-only change.

Validation

• git diff --check

I did not run the Sphinx docs build because this only changes a short prose block and one code-comment line.

Notes

• Commit includes Signed-off-by for the DCO/OCA process.
• No issue was opened because this is a narrow documentation clarification.

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[arizedatngo]

@sonleoracle @paul-cayet I opened this as a docs-only update to clarify the Arize/Phoenix tracing path and related evaluation references. You both have recent history on this tracing doc; could one of you take a look or route it to the right reviewer once the OCA gate is clear?

[cesarebernardis]

@arizedatngo Thank you for the contribution!

To start, could you please sign the Oracle Contributor Agreement (OCA)?

You can find more info in the comment at #235 (comment)