docs: Add English and Japanese README

[cuic19053-hue]

🌐 Add Multilingual README (English & Japanese)

Motivation

This project is an outstanding Chinese learning roadmap for AI Agent full-stack engineers (580,000+ words, 110 tutorials, 194⭐). However, the README is only available in Chinese, which limits its reach to the international community.

This PR adds English and Japanese translations of the README to help non-Chinese-speaking learners discover and benefit from this roadmap, broadening the project's international impact.

Changes

File	Change
README.md	Added language switcher (简体中文 | English | 日本語) at the top
README.en.md	New — English translation
README.ja.md	New — Japanese translation (Keigo です/ます style)

Translation Strategy

• ✅ Preserve all Markdown structure (headings, tables, lists, blockquotes, code blocks, dividers)
• ✅ Keep all relative paths (全栈学习路线/...) unchanged so internal links stay valid
• ✅ Keep all URLs unchanged (shields.io badges, GitHub issues/discussions, Obsidian, Creative Commons)
• ✅ Preserve ASCII art — only translate the Chinese text inside, keep box-drawing characters (═ ║ ╪ ╔ ╗ ╚ ╝)
• ✅ Localize badge labels to each language for a native reading experience
• ✅ Add language switcher at the top of all three README files
• ✅ Honor CC BY-SA 4.0 license — retain attribution to Karovia (original author)

Terminology Examples

中文	English	日本語
全栈工程师	Full-Stack Engineer	フルスタックエンジニア
学习路线	Learning Roadmap	学習ロードマップ
毕业项目	Graduation Project	卒業プロジェクト
费曼学习法	Feynman Technique	ファインマン学習法
造轮子	Reinventing the Wheel	車輪の再発明

License Compliance

This repository is licensed under CC BY-SA 4.0. These translations are derivative works under the same license, with full attribution to the original author Karovia.

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🙏 Thank you for maintaining this valuable resource! If any translation needs adjustment, I'm happy to revise.

ℹ️ Note: Translations are AI-assisted and carefully reviewed. Please let me know if you spot any unnatural phrasing or terminology preferences — I'll update accordingly.