docs: Add Japanese translation for documentation by demouth · Pull Request #1740 · php/frankenphp

This PR adds comprehensive Japanese translation for all FrankenPHP documentation files.

All 19 documentation files have been translated into Japanese with focus on:

  • Natural Japanese readability while maintaining technical accuracy
  • Consistent terminology throughout all files
  • Preserving line correspondence with English version for easier maintenance

This will help Japanese developers better understand and utilize FrankenPHP.