Clarify which options are `book` options and which are document options in books docs

[cwickham]

From #4936, it's hard to know where some of the options on the book structure page should go, e.g. section numbers:

number-depth: 1

go under a format or in a document, not under book.

Consider some combination of the following:

• Could use filename to disambiguate: using _quarto.yml for book options, and chapter-1.qmd for document level yaml.
• Could include more context in all YAML snippets: e.g. always explicitly include option hierarchy starting at

  book:   
     
  

  for book options.
• Be explicit in Overview that some options are book ones and some are relevant format ones, and describe conventions used in examples.
• Be more explicit with the introduction of each example whether something is a book option or format option.
• Reorder page to first talk about all relevant book options, then talk about format options. Be explicit about this ordering in Overview.