move NEWS.rst into docs/, add to readthedocs build

[warner]

While wrangling the 1.12.0 release, I discovered that NEWS.rst isn't as easy to reference as our other docs, because it doesn't live in docs/ (and thus cannot be part of the Sphinx build that gets published on readthedocs.io).

So the task is to move it down into docs/, update the index to include it, and update any pointers (maybe in README, maybe on the web site / wiki). And also to check on the formatting, make sure things render properly (but are mostly readable as plain text).

[exarkun]

Moving open issues out of closed milestones.

[meejah]

Ticket retargeted after milestone closed