db_config.md: add ALTER SYSTEM parameters

Summary

• db_config.md: add ALTER SYSTEM parameters

Address changes from picodata@2b65dee1

24.7.0-978-g2b65dee1

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• Close #406 (closed)
• Staging: https://docs.binary.picodata.io/picodata/asabirov_add_alter_system_parameters/reference/db_config/