DOCS-10810 - LLM Observability Experiments

[cswatt]

What does this PR do? What is the motivation?

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

TODOS:

• Link for joining the preview
• Link for Experiments Postman collection

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/cswatt/llm-datasets-experiments/llm_observability/experiments/

[cswatt]

Tracking docs review with DOCS-11097

[urseberry]

This is impressively detailed. I'm glad we talked about it in the meeting before I reviewed the actual PR. It's a bit dense, and I think I would have had trouble understanding the context without your explanation.