fix: improve otel fanout docs discoverability [closes DOC-796]

[Kathryn May (katmayb)]

Description

This updates the OpenTelemetry tracing documentation so users can discover the recommended OTEL fanout approach via search and understand when to use it versus the collector telemetry guide.

Changes made:

• Added SEO/frontmatter description to langsmith/trace-with-opentelemetry that explicitly includes LANGSMITH_OTEL_ENABLED and OTEL fanout terminology.
• Added explicit body copy near the top of the page mentioning OTEL fanout and LANGSMITH_OTEL_ENABLED to improve on-page search relevance.
• Renamed the advanced section heading to use fanout wording and added guidance clarifying:
  • use LANGSMITH_OTEL_ENABLED=true for application tracing fanout
  • use langsmith-collector for self-hosted LangSmith platform infrastructure telemetry

Resolves DOC-796.

Test Plan

• Confirm /langsmith/trace-with-opentelemetry includes searchable terms: OTEL fanout and LANGSMITH_OTEL_ENABLED.
• Verify the page now explains when to use OTEL fanout vs /langsmith/langsmith-collector.
• Run make format and make lint (currently blocked by existing repository-wide lint failures unrelated to these doc changes).

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❌ docs.json validation failed

This PR has issues with docs.json: either pages were removed without redirects, or some pages reference non-existent source files.

 The following pages were removed from docs.json without adding redirects:
  - oss/javascript/langchain/test
  - oss/python/langchain/test

Please add a redirect for each removed page to the `redirects` array in docs.json.
Example: {"source": "/path/to/removed-page", "destination": "/path/to/new-location"}

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