Improve documentation and tutorials

Improve the tutorial and in-app documentation.

No specific files, entry points, or tests are listed. Start by browsing the repository for documentation sources (e.g., README.md, docs/ content, and in-app help/tutorial screens) to find where tutorial and user guidance are maintained. Then review related style/guideline files before editing, and use any docs build/lint command if present to verify formatting. Done means the tutorial and in-app documentation content has been improved in explicit files and the changes are ready for review.

索引モデルが issue の本文から書いたものです。