arrow

ARROW-5452: [R] Add API documentation website (pkgdown)

I've deployed a test page at https://enpiar.com/arrow-r-site/ (source: https://github.com/nealrichardson/arrow-r-site) containing the result of building the pkgdown site.

There's a lot of work to be done to flesh out the docs to make them actually useful, but this patch gives us the foundation and makes very visible where the docs are inadequate.

One thing I'd like to do before finishing this patch is add the necessary scripts etc. to `/dev/gen_apidocs/` so that the R docs are built alongside the js and java docs.

Author: Neal Richardson <neal.p.richardson@gmail.com>

Closes #4419 from nealrichardson/pkgdown and squashes the following commits:

78e7c07f <Neal Richardson> Merge upstream/master
4bd72c47 <Neal Richardson> Merge upstream/master
85880667 <Neal Richardson> Merge upstream/master; fix missing license
af08c3a1 <Neal Richardson> A little doc consolidation
f2129c6a <Neal Richardson> R doc tweaks
141dee5d <Neal Richardson> WIP pkgdown site (R docs)
9dac96d9 <Neal Richardson> Run coverage stats for R on Travis-CI
0cdd141b <Neal Richardson> Merge upstream/master
e21732ac <Neal Richardson> Revise README to include more thorough installation and other dev instructions