Consider relocating NEWS/What's New docs

[erlend-aasland]

Also: should we think about moving this information to a different file altogether? This information isn't just relevant to core developers; it's relevant to any contributors.

It's possible (and easy) for core developers to add NEWS entries to other contributors' PRs, but few core devs do so; the de-facto expectation is that contributors should write their own NEWS entries

Originally posted by @AlexWaygood in https://github.com/python/devguide/pull/1238#pullrequestreview-1768007404