[Translations]Add Japanese translation link to all README versions

[nwiizo]

📝 Description

Related to #61
/kind documentation

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

I hope this message finds you well. As we continue to enhance the accessibility of the "Platforms on Top of Kubernetes" book, I'm pleased to submit this PR to add the Japanese translation link to the other language versions of the README. 🌐

This small but significant change will help increase visibility of the Japanese translation and improve navigation for our multilingual readers. 🧭

🔄 Changes

• ➕ Added the Japanese translation link to all README files using the following shell script:

  find . -name 'README.md' -type f -exec sed -i '' 's/\(_🌍 Available in_: \[English\](README\.md).*\)/\1 | [日本語 (Japanese)](README-ja.md)/' {} +

• 🔍 Ensured consistent formatting with existing language links
• 🌐 Verified that the link correctly points to the Japanese README (README-ja.md)

📌 Notes

• 🎯 This update aims to create a more interconnected experience for readers across all language versions
• 🌏 By adding this link, we're further supporting the global Kubernetes and cloud-native community
• 💬 I welcome any feedback or suggestions for improvement in how we present language options
• 🛠️ The use of the shell script ensured consistency across all README files and minimized the chance of manual errors

Thank you for your continued support and dedication to making this valuable resource accessible to a global audience. Your efforts in maintaining and improving this project are truly appreciated. ✨

I look forward to your review and am happy to make any necessary adjustments. 🙏

[salaboy]

this is perfect!