Bump to version 0.4

[caleb-johnson]

Remaining action items:

• Update version [sub]string in all files where it appears in the repository
  • .mergify.yml
  • pyproject.toml
  • .github/workflows/docs.yml
• move release notes to 0.4 directory
• [-] merge everything except this PR on the milestone
• build docs locally; proofread the release notes
• merge this PR
• tag the release
  1. git checkout main
  2. git pull
  3. git show (verify it's the commit we want to tag)
  4. git tag -a 0.4.0 -m 0.4.0
  5. git show (double/triple check the tag is on the correct branch)
  6. git push origin 0.4.0
• Wait for CI to finish. The release should then be available on GitHub and pypi.
• create the stable/0.4 branch
  1. start with same commit as above checked out
  2. git checkout -b stable/0.4
  3. git show (check before pushing)
  4. git push --set-upstream origin stable/0.4
• get docs to deploy from the new stable branch (should happen automatically at this point after CI runs both on the new stable branch and then on the gh-pages branch)
• [-] move remaining old milestone items to new milestone and close old milestones