Add Node.js experiment setup instructions

[mehulsonowal]

What does this PR do? What is the motivation?

Update the Agent Observability Experiments documentation to include Node.js instructions alongside Python and organize the setup content under Guides.

• Add Python/Node.js selectors for setup and experiment code snippets.
• Document Node.js tracer initialization, datasets, tasks, evaluators, summary evaluators, OpenTelemetry spans, and experiment runs.
• Move the setup and Experiment topic pages into the Guides section.
• Update navigation, links, and legacy aliases.

Merge readiness

• Ready for merge

Testing

• git diff --check
• Local Hugo preview verified for the updated routes.

AI assistance

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/guide/experiments

Removed or renamed files (these should redirect)

• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/improve/experiments/setup

Renamed files

• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/guide/advanced_experiment_runs
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/guide/analyzing_experiment_results
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/guide/experiments_api
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/guide/prompt_optimization

Modified Files

• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/guide/
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/improve/experiments/
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/improve/playground
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/instrument/otel_instrumentation
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/investigate/evaluations/external_evaluations/deepeval
• https://docs-staging.datadoghq.com/docs/nodejs-experiments-setup-signed/llm_observability/investigate/evaluations/external_evaluations/pydantic

[buraizu]

Created DOCS-15714 for the editorial review.