Add english transaction for project structure doc (separateexpress)

### Description

I noticed that multiple language versions are available for `separateexpress`, but the English version is missing from the documentation.

### Why this matters

Since this repository aims to provide Node.js best practices to a global audience, having an English version of the project structure documentation would improve accessibility and usability for a wider community.

### Suggested solution

* Add an English translation of `separateexpress.chinese.md` (as an example; other language variants exist as well)
* Follow the existing naming convention used for English files (e.g., `separateexpress.md`)

### Contribution

I’d be happy to help with the translation if this is something you’d like to include.

Start by reading the existing language variants, especially separateexpress.chinese.md, and check the repository’s naming convention for English documentation files. Translate the project-structure content into separateexpress.md, preserving the documented structure and formatting. Done means the English file exists alongside the other language versions and is complete and readable.

Written by the indexing model from the issue text.