Enhance documentation for advanced experiment runs

[CFLJacquet]

Updated description to specify running experiments on a subset of the dataset. Added a new section on running experiments on a subset of the dataset with examples.

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Modified Files

• https://docs-staging.datadoghq.com/CFLJacquet/dataset_subset/llm_observability/experiments/advanced_runs

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[chatgpt-codex-connector]

Assign filtered dataset before running experiment

The example calls LLMObs.pull_dataset(...) without storing its return value, so following these steps can leave users running LLMObs.experiment(..., dataset=dataset) with an unfiltered dataset variable from earlier code. In the subset-run workflow this defeats the intended tag filter and can unexpectedly execute (and bill for) the full dataset.

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