Navigation panel should display pages if they can inherit content and still exist

We can also remove all special handling of the navigation panel and let all XWiki pages be displayed in the panel, regardless of their type: book pages, book content subpages, variants, etc.

Then, we need to make sure that navigation is understandable for the editors, assuming that book editors master the complex logic of building a book (they know that content subpages exist, etc.)

Note also that some of these xwiki pages are hidden, so not everyone would see them.