[Translation] Add Japanese version of main

[nwiizo]

📝 Description

Continues work on #61
/kind documentation

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

🔄 Changes

• ➕ Integrated index content into README-ja.md
• 🔍 Ensured consistency in formatting and style with both the original English version and all previous translations
• 🔄 Updated the table of contents in README-ja.md to include index

📌 Notes

• 🎯 As with all previous chapters, I've maintained a balance between accuracy and natural Japanese expression
• 🌏 This translation continues to expand the resources available to the Japanese-speaking Kubernetes and cloud-native community
• 💬 I warmly welcome any feedback or suggestions for improvement, which will be crucial as we approach the project's completion
• 🚀 I will be working on the remaining tasks, including [brief description of additional tasks], and will keep you updated on the progress