Create a changelog file

[andrius-puksta-sensmetry]

Create a changelog CHANGELOG.md that lists what each PR does. This makes it easy to incrementally update it in every PR and avoid needing to collect the notes from the commit log when a release is made.

[consideRatio]

Create a changelog CHANGELOG.md that lists what each PR does.

Since this project has published docs associated with it already, and handles markdown, lets put the changelog there.

This makes it easy to incrementally update it in every PR and avoid needing to collect the notes from the commit log when a release is made.

The practice of incrementally updating a changelog in every PR is something I've seen fail several times over - it leads to regular tiresome merge conflicts and toil that don't bring any value by itself. I'm very opinionated that we avoid it!

Proposal

1. Let's use github-activity for now to generate changelog entries based on merged PRs titles since last tag, categorized by their labels.
2. Let's be thoughtful on our PR titles, making titles reasonable for a changelog, with technical details focused towards PR descriptions.
3. Let's update RELEASE.md to not just bump the version but also write a changelog entry.

Example from jupyterhub's RELEASE.md

1. Create a PR updating `docs/changelog.md` with [github-activity] and
   continue when its merged. For details about this, see the [team-compass
   documentation] about it.

   [team-compass documentation]: https://jupyterhub-team-compass.readthedocs.io/en/latest/practices/releases.html