system_tables.md: two updates

Summary

• system_tables.md: remove owner from _pico_index #292 (closed)
• system_tables.md: _pico_table description #289 (closed)

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• Close #289 (closed), #292 (closed)
• Cherry-pick to: none / 24.4 / 24.3 / 24.2
• Staging: https://docs.binary.picodata.io/picodata/rosik_misc/architecture/system_tables.md