Update the Hugo building instructions

[maelle]

https://github.com/r-lib/hugodown/

• Change output in Rmd archetype.
• Tell contributors to install hugodown if they want to contribute a post from Rmd.

Contra: a package to install. Pro: no more "there will be an index.html, ignore that" since only a index.md will be produced!

[steffilazerte]

@maelle, I'm just going through some of the older issues. Is this still relevant?

[maelle]

I'd like to keep this issue opened but I've changed my mind on what's the goal.

One thing I'd like us to have is a command contributors can run if they want to build the website locally with our Hugo version. Especially important if they have another Hugo version for their own website.

• In hugodown it means running hugodown::hugo_install() with the version number from our Netlify config, then hugodown::hugo_start() will use it.
• We can't recommend blogdown yet because of https://github.com/rstudio/blogdown/issues/611 Once that issue is solved we can recommend blogdown::build_site(). I'll update things here when it's possible.

[maelle]

@steffilazerte see c99291a (#182)