Rename no-section-label to be a positive flag e.g. section-label-toc

[szabgab]

Problem

IMHO negative configuration options are confusing as they use double negative no-section-label defaults to false. It would be better to rename the option to something positive. e.g. section-label-toc.

I suggest including the -toc part to indicate that this is about the table of contents.

This would allow for another option called section-label-title that would make mdbook add the numbers to the titles for #2760

Proposed Solution

1. Add the new flag and add a deprecation warning for the no-section-label flag. Update the documentation to indicate the new flag and the deprecation.
2. Open an issue to remove the new flag when breaking changes are allowed.

Notes

No response