Sort out favicons

[alice-hannah]

Browser header and tab icons still showing objectiveR name

[alice-hannah]

Think issue with old favicons fixed in #63. Done by deleting pkgdown/favicons directory and recreating using pkgdown::build_favicons(). Deleted dev folder in gh-pages branch, and rerun GHA to build site. May need to repeat this when publishing new pkg version.

A different issue has arisen where favicons generated aren't quite right - only top corner of logo used. Needs more work to resolve. (See pkgdown PR - r-lib/pkgdown#2804)