Add advice to use `usethis::edit_pkgdown_config()`

[olivroy]

If pkg is not available within the functions

Added a couple of call to improve error messages as I saw them. Sorry the PR is not very focused. Sometimes it is hard to come back to it later...

Also the many changes in snapshots are caused by the fact that {.file } quotes in non-interactive, while cli::style_hyperlink() doesn't.

Addresses #2329