Generate jb-books TOC with parts and not chapters

[ketozhang]

Context

Currently jb toc mynotebook -f jb-books genenates a toc file with chapters and sections and ignores using parts.

Proposal

Add option in jb toc to start with parts

Suggestions:

1. Maybe add the formats: jb-book-chapters, jb-book-parts (for backward compatibility jb-book defaults to jb-book-chapters.

Tasks and updates

No response

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