Convert CHANGELOG and CONTRIBUTING from .rst to .md

[hugovk]

Markdown is easier to work with, these files are not used by Sphinx, and this will make it easier to copy and paste the generated changelog from GitHub into the file (and avoid mistakes like https://github.com/python/python-docs-theme/pull/276#discussion_r2461062889).

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📚 Documentation preview 📚: https://python-docs-theme-previews--277.org.readthedocs.build/

[AA-Turner]

I think to an extent this is churn, and I do disagree with the premise. I would suggest that a consistency of markup formats within the project matters more than copying-and-pasting into a third party tool (IIRC, GitHub copes with pasting in the rendered HTML, though one has less control).

This seems to be a loosing battle though, and README files have been changed to markdown for various reasons elsewhere, so I won't block this if there's desire to push ahead.

A