[Translation] Add Japanese version of Chapter 6

[nwiizo]

📝 Description

Continues work on #61
/kind documentation

Dear @salaboy and the platforms-on-k8s team,

Thank you for your continuous support and timely reviews of the previous chapters' translations. I sincerely appreciate your dedication. 🙏

This PR adds the Japanese translation for Chapter 6 of the "Platforms on Top of Kubernetes" book. 📚

I apologize for submitting PRs in quick succession. I understand this may increase your workload, and I'm grateful for your patience and diligence in reviewing these translations. 🙇‍♂️

🔄 Changes

• ➕ Added Chapter 6 content to README-ja.md
• 🔍 Ensured consistency in formatting and structure with the original English version and other translations
• 🔄 Updated the table of contents in README-ja.md to reflect the addition of Chapter 6

📌 Notes

• 🎯 As with previous translations, I've focused on maintaining accuracy while ensuring the content is accessible to Japanese readers.
• 🌏 This continued effort aims to further benefit the Japanese-speaking Kubernetes and cloud-native community.
• 💬 I welcome any feedback or suggestions for improvement. Please don't hesitate to point out any areas that need refinement.

Thank you for your continued support and for taking the time to review these translations. Your feedback is invaluable in ensuring the quality of the Japanese version. ✨