docs(i18n): update it README to match ja/en structure (#275)

- Restructure document to follow ja/en SSOT format
- Add Profile Management UI section
- Update image paths from demo-it.png to it/demo.webp
- Add new images: first-ux.webp, identity-management.webp
- Unify terminology: identità → profilo
- Add detailed SSH explanation and troubleshooting sections

🖥️ IDE: [Cursor](https://cursor.sh)
🔌 Extension: [Claude Code](https://claude.ai/download)


Model-Raw: claude-opus-4-5-20251101

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: nullvariant