[Sync to prerelease] Don't imply different behaviour for chapter numbering in index.qmd

docs/books/book-structure.qmd

@@ -22,7 +22,7 @@ book:
     - references.qmd
 ```
 
--   The `index.qmd` file is required (because Quarto books also produce a website in HTML format). This page should include the preface, acknowledgements, etc. and headings in the `index.qmd` file are unnumbered by default. The HTML version of the book will use the `index.qmd` as the home page and if provided, will place the `cover-image` on that page.
+-   The `index.qmd` file is required (because Quarto books also produce a website in HTML format). This page should include the preface, acknowledgements, etc. The HTML version of the book will use the `index.qmd` as the home page and if provided, will place the `cover-image` on that page.
 -   The remainder of `chapters` includes one or more book chapters.
 -   The `references.qmd` file will include the generated bibliography (see [References] below for details).
 
@@ -46,10 +46,10 @@ In the absence of a level-one header or a title set in the YAML front matter, th
 
 ## Chapter Numbers
 
-All chapters are numbered by default. If you want a chapter to be unnumbered simply add the `.unnumbered` class to its main header. For example:
+All chapters are numbered by default. If you want a chapter to be unnumbered simply add the `.unnumbered` class to its main header. For example, it is common to omit the chapter number for `index.qmd`:
 
-``` markdown
-# Resources {.unnumbered}
+```{.markdown filename="index.qmd"}
+# Preface {.unnumbered}
 ```
 
 You can mix together numbered and unnumbered chapters. Note however that while you can link to unnumbered chapters, you can't [cross reference](/docs/authoring/cross-references.qmd) figures, tables, etc. within them. Unnumbered chapters are therefore mostly useful for prefatory content or references at the end of your book.