docs: simplify Docker setup for Windows, Linux, and macOS

[dadachann]

Summary

• Explicitly include Docker Desktop and PowerShell guidance for Windows users.
• Use shared multiline commands without a mandatory Unix installation path.
• Keep the Linux restricted-directory warning and mirror replacement before startup.
• Remove token-generation and runtime details from the beginner flow; retain a short firewall reminder.
• Keep all three locales aligned and regression-test the portable command blocks.

Validation

Node 24: npm test, typecheck, full build and static-output tests; git diff --check.
No end-to-end Windows runtime verification is claimed.