DOC: Add release notes template for 2.3.1
Starting the release notes for 2.3.1, so we can start adding things (for new PRs, but will also follow-up with moving some things from 2.3.0 to 2.3.1)
@rhshadrach I remember you were adding those in a PR with a bug fix. I think we should better merge this, but since your PR will conflict, I'll leave it to you. Thanks @jorisvandenbossche 