Move release notes to the docs.
Review Request #10591 — Created June 14, 2019 and submitted
NEWSfile was getting long and difficult to maintain. To help
with both reading and writing the release notes, this change has moved
them all to the new place.
The content of the release notes haven't changed, with the exception
that references and ReST roles have been added where appropriate, to
just help take advantage of the fact that they're now being rendered to
Generated the docs. Went through every single release notes page to make
sure things looked correct.