system_tables.md: update _pico_index

Изменения

https://docs.binary.picodata.io/picodata/asabirov_edit_pico_index/architecture/system_tables/

Версия: Picodata 24.3.0-65-g6043629f

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• Close #225 (closed)
• Target branch: main
• Changes should be cherry-picked to 24.2: no
• Staging: https://docs.binary.picodata.io/picodata/asabirov_edit_pico_index/
• Follow-up for picodata!929 (merged)