[Translation] Add Japanese version of Chapter 7

[nwiizo]

📝 Description

Continues work on #61
/kind documentation

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

I hope this message finds you well. Your ongoing support and prompt reviews of the previous chapters' translations have been invaluable. I'm truly grateful for your commitment to this project. 🙏

This PR introduces the Japanese translation for Chapter 7 of the "Platforms on Top of Kubernetes" book. 📚

I understand that the frequent submission of PRs may be increasing your workload, and I deeply appreciate your patience and thoroughness in reviewing these translations. 🙇‍♂️

🔄 Changes

• ➕ Incorporated Chapter 7 content into README-ja.md
• 🔍 Maintained consistency in formatting and structure with both the original English version and previous translations
• 🔄 Revised the table of contents in README-ja.md to include the newly added Chapter 7

📌 Notes

• 🎯 As with earlier translations, my focus has been on preserving accuracy while making the content relatable to Japanese readers.
• 🌏 This ongoing effort is aimed at expanding the resources available to the Japanese-speaking Kubernetes and cloud-native community.
• 💬 I'm open to any feedback or suggestions for enhancement. Please feel free to highlight any areas that could benefit from improvement.

I extend my heartfelt thanks for your continued support and the time you dedicate to reviewing these translations. Your insights are crucial in maintaining the quality of the Japanese version. ✨