Update Japanese README

[s2015-turtle]

Summary

• align the Japanese README with the current English README and implementation
• document the private-protocol and 1 Mbit/s SocketCAN prerequisites
• clarify gear_ratio, startup attempts, watchdog defaults, and shutdown behavior
• remove internal Type-number, DDS worker, and queue implementation details

Why

The Japanese README predated the latest user-facing documentation cleanup. Some wording could be misread, particularly the transmission ratio and retry semantics, while important setup requirements were missing.

User impact

Japanese-speaking users now receive the same setup, safety, supported-model, and command-mode guidance as readers of the English README.

Validation

• compared parameter names and defaults with driver_config.cpp
• compared lifecycle, recovery, and shutdown behavior with driver.cpp
• checked model-profile values against robstride_motor_profiles.xacro
• ran git diff --check

Documentation-only change; no local build was run.