Mac OS X was renamed to OS X in 2012, and then renamed again to macOS in 2016.
I suggest that the current name (macOS) should be used whenever referencing the Apple OS.
The coding-conventions document covers useful introductory guidance for
all. This commit moves it out of the devel/ detail directory and into the
contributor/guide, fixing up links from other doc's in this repo and
leaving a cookie crumb of a document pointing at the new location.
Signed-off-by: Tim Pepper <tpepper@vmware.com>