add a changes.txt file to pull requests which will (roughly) list changes made in this pull compared to the stable branch, for beta PRs.

• script that can diff against stable and generate changes.txt when run, using commit messages or titles
• change gh-action workflow to run the script when PR is made to stable
• improve changelog format using more useful information, such as the original PR body

references:
https://stackoverflow.com/questions/33641307/how-to-get-git-diff-to-only-show-commit-messages
(curl for pr body): https://stackoverflow.com/questions/61584664/how-to-get-pull-request-message-body