docs: add OpenTelemetry provider guides

[antonvishal]

Summary

Add Stagehand v4 OTLP guides for Axiom, Braintrust, Honeycomb, HyperDX, Laminar, Langfuse, LangSmith, Sentry, SigNoz, and the OpenTelemetry Collector.

I verified the added providers manually.

Follow-up: I will add a Stagehand page in each provider's own docs.

Test plan

• pnpm --filter @browserbasehq/stagehand-docs check

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Summary by cubic

Adds OpenTelemetry provider setup guides for Stagehand v4 so users can send browser traces to Axiom, Braintrust, Honeycomb, HyperDX, Laminar, Langfuse, LangSmith, Sentry, SigNoz, or an OpenTelemetry Collector.

New Features

• Adds an overview page describing the Stagehand trace format and how to choose a backend.
• Adds provider logos and a card linking to the overview from the observability configuration page.
• Registers the new pages in the docs sidebar navigation.

^{Written for commit f742c84. Summary will update on new commits.}

[changeset-bot]

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Latest commit: f742c84

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[github-actions]

This PR is from an external contributor and must be approved by a stagehand team member with write access before CI can run.
Approving the latest commit mirrors it into an internal PR owned by the approver.
If new commits are pushed later, the internal PR stays open but is marked stale until someone approves the latest external commit and refreshes it.