docs(admins): add Tracing setup guide (Braintrust / Langfuse)

[justin-tahara]

Description

Adds a customer-facing setup guide for Tracing — connecting Braintrust / Langfuse to send LLM call traces from a self-hosted Onyx deployment for monitoring and evaluation.

Documents the corresponding feature PRs in onyx-dot-app/onyx (#12529, #12535, #12539).

• New page: admins/advanced_configs/tracing.mdx (under Advanced Configs).
• Covers: connecting each provider from Admin Panel → Usage → Tracing (with where to get keys, optional project/region/base-URL fields), disconnecting, and the environment-variable equivalents for backwards compatibility.
• Notes it's self-hosted only (not configurable on Onyx Cloud) and that UI changes apply within ~30s without a restart.

Notes

• Text-only for now (no screenshots) — happy to add dashboard images (matching the Web Search page style) if wanted.
• Placed under Advanced Configs alongside hook_extensions; can move to Auditing/Analytics if that's preferred.

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Project	Status	Preview	Updated (UTC)
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