SCR v3.1 Release Notes

[gonsie]

• [x] Merge any outstanding PRs, close related issues
• [x] Pass GitLab CI tests
• [x] Update version numbers code
• [x] Update docs dependency versions
• [x] Create docs for new python interface
• [x] Create a PR with these changes
• [x] Merge the PR
• [x] Tag a release on GitHub
• [ ] write some release notes
• [ ] Update the spack package: update hash for download; update dependency versions

See https://github.com/orgs/LLNL/projects/26/views/1 for tracking

[gonsie]

@kathrynmohror Is it okay if we go to v4.0?

[kathrynmohror]

@gonsie Are there user facing interfaces that have changed or been added? If so, then yes

[CamStan]

PR for Spack update: https://github.com/spack/spack/pull/45737