Add favicon

[Bisaloo]

I was looking for an opened devguide tab but couldn't find it because it has no favicon.

It might make sense (and it would be useful!) to add the rOpenSci favicon.

[maelle]

Thank you!

I think they are not supported by bs4_book() yet https://github.com/rstudio/bookdown/issues/1011

Now you'll have to open less tabs :stuck_out_tongue_winking_eye: (just kidding).

[mpadge]

Can't do anything here until upstream changes to bookdown linked to above, so bumping milestone to 0.9 for now.

[maelle]

Solved in the dev version, with Quarto https://devdevguide.netlify.app/

Cf https://quarto.org/docs/websites/website-tools.html#social-metadata