[codex] add documentation book usage guidance

[ootakazuhiko]

What changed

• added usage, license, and update guidance to the top page
• normalized Issue / PR wording in the prerequisites section

Why

• the reader-facing top page still lacked direct guidance for reuse, update tracking, and license handling

Validation

• git diff --check
• bundle exec jekyll build --source docs --destination _site
• node ../book-formatter/scripts/check-links.js docs
• node ../book-formatter/scripts/check-markdown-structure.js docs --fail-on error

[ootakazuhiko]

当初の docs/index.md 差分は、現時点では main に対して tree を変えない no-op 状態です。重複レビューを避けるため、この draft PR は close します。