Add docs or link to docs of how to specify a non Google font

[maelle]

(in the Customize article)

[maelle]

• what font "files" does one need.
• how to indicate their usage in the pkgdown config.

I haven't found a good example in the sass docs.

[hadley]

Docs at https://rstudio.github.io/sass/reference/font_face.html

[maelle]

In rotemplate what worked:

• Only mentioning the font face in the pkgdown config https://github.com/ropensci-org/rotemplate/blob/3ac30a939a3aab97c3ca78c7e1d4672952b6ef45/inst/pkgdown/_pkgdown.yml#L22
• Adding scss rules https://github.com/ropensci-org/rotemplate/blob/985ffb56006476015b4027b20b518fead9c37840/inst/pkgdown/extra.scss#L189-L200= (in a way I think it was more natural anyway as I was copy-pasting from another (s)css file) + making sure it was possible to load the fonts from another website https://github.com/ropensci/roweb3/blob/9a4ad99d8217cd750e796cd8291cdd00dd14c25c/netlify.toml#L24-L28

From the docs linked in #1968 (comment) I imagine one could use the src field?