[R][Docs] Add docs on what dplyr + tidyverse functionality we support

It's impossible to know what we do or do not support without literally trying it out.  Other things to consider:

* "I think it would be hella useful to have some kind of document giving a view into the arrow package’s coverage of all things dplyr. This would be great for users and also as a project management kind of tool"

• "and you could print that in an rd page...look into what dbplyr does: it has rd files for its dplyr methods https://dbplyr.tidyverse.org/reference/index.html...it will be trickier for us because we don't export them, so you'll have to do more @roclet fun and probably pkgdown.yml work to make them show up"

Perhaps this could work as a longer form vignette and a shorter form rd file which prints out all the .rd functions?