docs: arize phoenix observability

[s-yeddula]

Hi Mem0 team - I’m Sanjana from the Arize Phoenix team. Under the open-source docs, I added an Observability section and how to trace using Phoenix page. I also included an example of tracing one of your provided examples which I believe will be helpful. This PR adds Phoenix as one observability option.

Thank for taking a look - let me know if you have any questions or desired changes!

Sanjana

Fixes # (issue)

Type of change

• [x] Documentation update

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have checked my code and corrected any misspellings

Maintainer Checklist

• [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
• [ ] Made sure Checks passed

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