docs(planning_data_analyzer): add displacement-based metrics note

[beomseok-kimm]

Summary

• add a displacement-based metrics note for planning_data_analyzer
• document ade, fde, ahe, fhe, lateral_deviation, and longitudinal_deviation
• explain how multi-horizon results are derived for full, 1s, 2s, 4s, and 8s

Testing

• not run (documentation change)

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