Updates to README and vignettes

This PR applies some copyediting to the documentation, specifically the README and vignettes.

README is more concise, and points to the getting started vignette more
(at least on GitHub itself) the README has a video snippet from my RStudioConf talk about piggyback
renamed intro.Rmd to piggyback.Rmd to take advantage of pkgdown's getting started feature.
- Applied a bit of the copyediting from README but is mostly the same otherwise.
- Added manual text-wrapping at approximately 80 characters to make it a little easier to work with (imo)
alternatives.Rmd is mostly the same
adds duckdb/arrow vignette (closes #111)

To Do:

~If I get to #97 before this PR is reviewed/merged, add pb_read/pb_write to piggyback.rmd~ will address in #115 after merging this

ropensci / piggyback