[DOC] Add Arize Phoenix Observability Docs

[Jgilhuly]

Description of changes

Summarize the changes made by this PR.
Adds Arize Phoenix as an observability integration. The docs link to a complete example, and show a happy path to log initial traces.

Let us know of any feedback there! I think I captured initial feedback - happy to add whatever else we need to make these top tier docs!

[github-actions]

Reviewer Checklist

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
• Can you think of any inputs or external events that could break the code? Is user input validated and safe? Have they been tested?
• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
• Should any logging, debugging, tracing information be added or removed?
• Are error messages user-friendly?
• Have all documentation changes needed been made?
• Have all non-obvious changes been commented?

System Compatibility

• Are there any potential impacts on other parts of the system or backward compatibility?
• Does this change intersect with any items on our roadmap, and if so, is there a plan for fitting them together?

Quality

• Is this code of a unexpectedly high quality (Readability, Modularity, Intuitiveness)