Add methodology section to multi-turn online evaluators page

[langsmith-fleet]

Fixes DOC-1183

Summary

• Adds a "How it works" section before the Prerequisites section on the multi-turn online evaluators page
• Explains the end-to-end evaluation lifecycle: trace ingestion, idle time detection, message assembly, LLM-as-a-judge evaluation, and feedback recording
• Addresses a request from #docs-plz to document the methodology at https://docs.langchain.com/langsmith/online-evaluations-multi-turn#prerequisites

Links

• Linear: https://linear.app/langchain/issue/DOC-1183/add-methodology-explanation-to-online-evaluations-multi-turn
• Slack: https://langchain.slack.com/archives/C09G1T60QV9/p1780072374303589

Verification

Not run; docs-only copy change.

Reviewers

Requested review from: Kathryn May (@katmayb), Florence Morris (@fjmorris)

[github-actions]

Mintlify preview branch generated: preview-docsad-1780072755-a0a75c0

Site preview: https://langchain-5e9cc07a-preview-docsad-1780072755-a0a75c0.mintlify.app

Preview links may take a few minutes to start working while the deployment finishes.

Changed documentation pages (preview deep links):

• src/langsmith/online-evaluations-multi-turn.mdx