Group deprecated functions together in `pkgdown` Reference Page

[jamesmbaazam]

It would be nice to add a section in the pkgdown Reference, similar to this section on superseded functions in dplyr and purrr. That will probably help users identify which functions are deprecated all in one place.

[sbfnk]

I agree, and probably also to move them all to R/deprecated.R

[sbfnk]

The lifecycle vignette recommends removing deprecated functions from pkgdown altogether (unlike superseded ones which aren't scheduled for removal) so will go with that when addressing #598.