feat: add support for base-url

[maelle]

Fix #8

@jranke two questions!

• would you have time to test/review this?
• can I list you as contributor (ctb) in the DESCRIPTION file? (which is independent of your answer to the first question!)

[jranke]

Hi, I tested this and it works very nicely when I use babelquarto::render_book("book_folder", site_url = "https://my.site/url") for deployment in CI, but also when specifying it in the YAML config. I avoid the latter, because then, when I preview in RStudio, I get links to the deployed version. So I prefer not to configure the site URL in the YAML config, even though the links across language versions are not functional when I build and preview in RStudio.

Yes, you can list me as contributor in the DESCRIPTION, thanks!