docs: add observability/langfuse to built-in-plugins + env-vars reference (#16929)
Documents the langfuse plugin shipped in #16917:
- website/docs/user-guide/features/built-in-plugins.md: new
observability/langfuse section (setup wizard vs manual, hook-by-hook
behaviour, verify / optional tuning / disable)
- website/docs/reference/environment-variables.md: Langfuse Observability
subsection under Tool APIs listing the 3 required + 5 optional env vars,
with a back-link to the built-in-plugins page
Validated: ascii-guard clean, npm run build succeeds, #observabilitylangfuse
anchor resolves.
Co-authored-by: teknium1 <teknium@users.noreply.github.com>
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