Add Japanese translation as tutorial_ja/ mirror

[turabe777]

Summary

Adds a Japanese translation under tutorial_ja/ as a self-contained mirror of
tutorial/.

Directory layout

tutorial/ # existing English (untouched)
tutorial_ja/ # new Japanese mirror
├── jupyter_notebook/ # 10 *_ja.ipynb
├── images/ # duplicated (~3.1 MB)
├── skills_ref/ # duplicated (~32 KB)
└── data/ # duplicated (~84 KB)

Why a full mirror

• Zero changes to tutorial/ — no risk to the English version.
• Self-contained relative paths — images/..., ../data/...,
  skills_ref/... resolve identically to the English version. Works on clone with
  no link rewrites.
• Easy to remove or extend — future locales (e.g. tutorial_ko/) can follow
  the same pattern.

Trade-off: ~3.5 MB of duplicated assets

Mostly images/ PNGs, plus byte-identical skills_ref/ Python files. I chose
duplication over relative ../tutorial/images/... links to keep the tree
self-contained. Happy to switch to symlinks if preferred.