system_tables.md: plugins

Picodata follow-up

• Follow-up for picodata!928 (merged)
• Doc pages to change: architecture/system_tables.md

New tables added: _pico_plugin, _pico_service, _pico_service_route.

See also: https://git.picodata.io/picodata/picodata/docs/-/blob/main/tools/dump_spec.lua