Feature Request: Configurable Starting Point for Chapter Numbering

[Wereldjimmy]

I would like to propose the addition of a configuration option that allows users to specify the starting point for automatic chapter numbering in Jupyter Book.

Currently, chapter numbering always begins at 1. However, in some books, it is customary to begin with Chapter 0 (e.g., for introductions or prefaces). Providing the option to define a custom starting chapter number would improve flexibility and alignment with various publishing conventions.

Additionally, this feature would be particularly valuable for appendices, which are often labelled alphabetically (e.g., Appendix A, with sections A.1, A.2, etc.) rather than using the standard numeric format. At present, there is no built-in way to automatically format all section numbers within an appendix to follow this pattern.

This starting point would then also be used in figure numbering, table numbering, etc.