Add chapter numbers to Jupyter Book

[jdebacker]

Summary

This PR updates the book structure so chapter numbering is generated automatically from the Jupyter Book / MyST table of contents, without hardcoding numbers into page titles.

What changed

• enabled project-level title numbering in book/myst.yml
• disabled numbering on the landing page in book/content/intro.md
• converted each top-level TOC part into a real page with its own short introduction:
  • book/content/background/index.md
  • book/content/using/index.md
  • book/content/repomgt/index.md
  • book/content/txteditor/index.md
  • book/content/appendix/index.md
• updated the TOC so those part pages act as the numbered top-level chapters and existing content pages are nested beneath them

Result

The numbering now behaves as follows:

• the title page is unnumbered
• top-level book parts are numbered 1, 2, 3, ...
• pages within each part are numbered as child chapters, for example 2.1, 2.2, ...
• numbers are generated from the TOC and will automatically update if pages are moved, inserted, or removed

Example

The book now renders part/chapter numbering in the style below:

• 1 Git and GitHub Background and History
• 2 Using Git and GitHub
• 2.1 Installing Git
• 2.5 Git and GitHub Collaborative Workflow
• 3 Open Source Repository Management

Notes

• This change introduces dedicated landing pages for each major TOC part, which currently use routes such as /index-1, /index-2, etc.
• The numbering behavior is now driven entirely by the book configuration and TOC structure rather than manual text edits.

Addresses Issue #16