* Set dbBook to true when traversing a chapter too.
Currently, a `<title/>` in a chapter and in a `<section/>` below that
chapter have the same level if they're not inside a `<book/>`.
This can happen in a multi-file book project. Also see the example at
https://tdg.docbook.org/tdg/4.5/chapter.html
Co-authored-by: Félix Baylac-Jacqué <felix@alternativebit.fr>
* Add docbook-chapter test
This tests nested `<section/>` and makes sure `<title/>` in the first
`<section/>` below `<chapter/>` is one level deeper than the `<chapter/>`'s
`<title/>`, also when not inside a `<book/>`.
Co-authored-by: Félix Baylac-Jacqué <felix@alternativebit.fr>
