system_tables.md: add _pico_index description

Summary

• system_tables.md: add _pico_index description

---

• Close #248 (closed)
• Cherry-pick to: none
• Staging: https://docs.binary.picodata.io/picodata/vbabayan_248_system_tables_md_pico_index_description/architecture/system_tables/#_pico_index