Attempt to add a favicon

[wlandau]

Re ropensci/drake#783 (comment), this PR attempts to add a favicon to the dev guide. Here, I adapt the steps that worked for drake's manual:

• Use pkgdown::build_favicon() to build a complete set of favicons from https://github.com/ropensci/dev_guide/blob/master/images/icon_short_color.png.
• Add a build step to tic.R to copy all the icons from images/ to _book/ after the book renders.

[wlandau]

Oh, and I forgot to add a third step: declare the favicon in the index.Rmd front matter (ref: fb03256). Now the favicon shows up when I build the dev guide on my local machine.

[maelle]

Thank you!!