Organize the documents and integrate them into the English version

In this change, I organized the documents under the doc directory.
Specifically, I performed the following tasks:

- Organized and consolidated the documents under doc/ja and doc/en.
- Deleted outdated documents.
- Reviewed the document structure and reorganized it into a more understandable configuration.

As a result, I believe document management has become easier and users can now find the information they need more readily.

Author: ydah