docs: add opik to traces examples

[vincentkoc]

Description

Adding new content on Comet Opik (https://github.com/comet-ml/opik) which is a leading open-source agentic evaluation, tracing and optimization platform. Our integration has been stable for a while and our users have asked for documentation, we are contributing example based on our reference implementations.

Type of Change

• Content update/revision

Motivation and Context

Users have requested impelemtation guides for Strands tracing.

Areas Affected

docs/user-guide/observability-evaluation/traces.md

Screenshots

n/a

Checklist

• I have read the CONTRIBUTING document
• My changes follow the project's documentation style
• I have tested the documentation locally using mkdocs serve
• Links in the documentation are valid and working
• Images/diagrams are properly sized and formatted
• All new and existing tests pass

Additional Notes

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.