Split up the Changelog

Documentation

Originally proposed in https://github.com/python/docs-community/issues/98

The full changelog is rather long, containing every NEWS entry since Python 3.5. We propose splitting it up into smaller individual files.

Doing this will also improve (parallel) builds -- build/NEWS (the file included by changelog.rst) is currently c. 55,000 lines long, the next longest file is 'only' 6,000 lines.

Consensus on the docs-community issue seemed to align with my proposal of splitting the changelog by minor version, meaning that we would have e.g. whatsnew/changelog/3.6.html.

I suggest that whatsnew/changelog/index.html would contain the entries for a pre-beta version (e.g. currently the 3.15 pre-alphas), but we could put these straight into whatsnew/changelog/3.15.html. Any strong opinions?

If we are happy to do this, I will propose the needed changes to blurb & then open PRs in CPython.

A