Add English translation of appendix-d.md for website.en

[Copilot]

This PR adds the English translation of appendix-d.md to the website.en folder structure, completing the internationalization of Appendix D.

Changes Made

• Created website.en/MarkDown/appendix-d.md with complete English translation of the Japanese original
• Translated all content from Japanese to American English while preserving the Q&A format
• Updated image references to point to the correct ../Images/ path within the English site structure
• Maintained all technical code examples and programming terminology accurately

Translation Details

The appendix contains coding guidelines for junior programmers in a Q&A format, covering:

• Clean code definition and importance
• Intent expression in code (WHAT vs HOW vs WHY)
• Method naming conventions from caller's perspective
• Single responsibility principle and method design
• Type inference (var) usage guidelines
• Debugging strategies for complex programs

All 8 questions and answers have been accurately translated to natural American English while preserving technical accuracy and the original structure. The translation follows established conventions used in other English appendices in the repository.

Validation

• ✅ UTF-8 encoding without BOM
• ✅ No Japanese characters remain in the translation
• ✅ All image links correctly reference website.en/Images/ folder
• ✅ Website functionality tested with local HTTP server
• ✅ Filename preserved as appendix-d.md
• ✅ Only modified files within website.en/MarkDown/ folder as required

Fixes #197.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Pull Request Overview

This PR adds the English translation of appendix-d.md to complete the internationalization of Appendix D within the website.en folder structure. The appendix contains coding guidelines for junior programmers in a Q&A format.

• Creates complete English translation of the Japanese appendix-d.md
• Translates 8 Q&A pairs covering clean code principles, method naming, and debugging strategies
• Updates image references to use correct English site structure paths

[Copilot]

[nitpick] The reference to 'Joshua Tree enlightenment' is unclear and may not be meaningful to readers unfamiliar with this specific reference. Consider providing a brief explanation or using a more universally understood analogy.

Suggested change

	- Making it recognizable by naming it (Joshua Tree enlightenment → Name and Conquer).
	- Making it recognizable by naming it (once you name something, you start to notice it everywhere—Name and Conquer).