Improve documentation for babelquarto YAML settings

[joelnitta]

It would be helpful to have documentation for the various fields of the babelquarto field that is added to _quarto.yml.

For example, here is the default one:

babelquarto:
  languagelinks: navbar
  languagecodes:
  - name: es
    text: "Version in es"
  - name: fr
    text: "Version in fr"
  - name: en
    text: "Version in en"
  mainlanguage: 'en'
  languages: ['es', 'fr']
title-es: title in es
title-fr: title in fr
description-es: description in es
description-fr: description in fr
author-es: author in es
author-fr: author in fr
lang: en

Why is the last lang:en needed? Isn't that redundant, considering that we already have mainlanguage: 'en'?

[Nenuial]

I'll also see if I can take this into account in the docs.

This is a quarto setting, having it set in _quarto.yml allows you to not repeat it in every document.

With profiles you could now set lang: es in a _quarto-es.yml for example and this setting then gets applied in every document when rendering the Spanish version.