Add vignette about local rendering

[joelnitta]

There is a vignette about rendering with CI, but it would be helpful to (first?) have one showing how to render locally.

This is basically two steps:

1. render_website()
2. servr::httw("_site") (or wherever you set up the website to be output)

[maelle]

Thank you @joelnitta!

Note that servr is mentioned in https://docs.ropensci.org/babelquarto/articles/babelquarto.html#previewing-your-multilingual-project

[Nenuial]

Thanks for the suggestion. Indeed, as @maelle mentioned, the use of servr for rendering is mentioned in the Get Started vignette. Do you feel that there should be a dedicated vignette for this?