docs(monitoring): add Opik monitoring integration guide

[vincentkoc]

Summary

• add an Opik monitoring integration tutorial page for Open WebUI docs
• document OpenTelemetry pipeline configuration pattern for Opik OTLP export
• link to Open WebUI Pipelines and Opik docs

Test Plan

• docs-only change
• verified frontmatter and markdown structure in docs/tutorials/integrations/monitoring/opik.md

[pr-validator-bot]

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