Option for content-dir in `_quarto.yml`

It's nice when writing a book to have a dedicated sub-dir for the "actual writing", as opposed to quarto scaffolding.

For example:

/
| -- quarto.yml
| 
...
|
| -- chapters/
|   -- chapter1.qmd

Currently, this works fine if your _quarto.yml looks like:

chapters:
  - chapters/chapter1.qmd

This gets unwieldy if you have lots of chapters, or a long dir name.

It would be nice to be able to specify a sub-dir once and then just use the chapter names, e.g.:

content-dir: chapters
chapters:
  - chapter1.qmd

My apologies if this is already possible and I just missed it in the docs.

So far, loving a bookdown -> quarto conversion. Everything feels super ergonomic. Thank you!

Start with the _quarto.yml examples in the issue and compare the current chapters paths with the proposed content-dir form. Trace how chapter paths are resolved, then verify that the documented configuration works for a book with multiple chapters.