Better document all the possible workflow since blogdown 1.0+

[cderv]

With the new version of blogdown, it is possible to do very advanced and custom workflow. From basic possibility (just with build_site() argument) to advanced one (using build.R and build2.R).

One shared user experience in #459 (comment) showed that the new possibilities of blogdown::build_site() are not easy to understand from current doc.

Currently we mentioned a bit of the new workflow in https://blog.rstudio.com/2021/01/18/blogdown-v1.0/

This could benefit a rework chapter in the book, or/and maybe a new vignette/article for the website.