[Translation] Add Japanese version of main

📝 Description

Continues work on #61 /kind documentation

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

➕ 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

🎯 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