architecture: update RPC API

Summary

• update proc_sharding description
• update system_tables (auto, via spec generator)
• describe new endpoint (proc_get_vshard_config)

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• Close #351 (closed), #355 (closed), #354 (closed)

• Cherry-pick to: none

• Staging:

  https://docs.binary.picodata.io/picodata/at_proc_sharding/architecture/rpc_api/

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