DOC add new what's new section to the changelog

[adrinjalali]

Adding a new section now that the release is done.

All existing PRs should move their changelog to this version now.

[BenjaminBossan]

Just wondering if we should call it "Unreleased" or something like that, in case we do a 0.3.1 release? Then rename it to the actual version once the release is made.

[adrinjalali]

If we do a 0.3.1, it'd be cherry picking commits from main to 0.3.X, and curating a custom changelog for it (moving from 0.4 to 0.3.1 in this case). We wouldn't just pull everything from main anyway.