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ARROW-8300: [R] Documentation and changelog updates for 0.17

This edits the NEWS.md and adds a python.Rmd (static) example of using the `reticulate` bindings.

cc @wesm @pitrou

Closes #6833 from nealrichardson/docs-0.17

Lead-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Co-authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>