Repair the Book Guide and connect every guide page to verified sources #117

Affected library: backtest
Current version: 0.1.8
Location: docs/book-guide/index.md and tutorial/user-guide pages

Problem

The published Book Guide names companion paths that no longer exist, and user-guide pages do not consistently point readers to the corresponding book explanations and notebooks.

Required outcome

Rebuild the Book Guide against the public Machine Learning for Trading companion repository and the book's current section titles. Add a concise "In the book" block to each tutorial and applicable user-guide page, stating what the referenced material adds. Keep the library guide useful to readers who do not own the book.

Acceptance criteria

• Every companion path in the Book Guide and per-page links resolves at the checked revision.
• A deliberately nonexistent companion path makes the documentation check fail.
• No book prose is copied into the library site, and the guide has no course-specific sales language.
• The strict MkDocs build passes.

Depends on the completed tutorial pages for final cross-links.