Release v0.3.1

[jtherrmann]

I confirmed that this version of the theme works with https://github.com/ASFHyP3/hyp3-docs/pull/424

[github-actions[bot]]

Developer checklist

• [x] Indicated the level of changes to this package by affixing one of these labels:
  • major -- Major changes to the API that may break current workflows
  • minor -- Minor changes to the API that do not break current workflows
  • patch -- Patches and bugfixes for the current version that do not break current workflows
  • bumpless -- Changes to documentation, CI/CD pipelines, etc. that don't affect the software's version
• [ ] (If applicable) Updated the dependencies and indicated any downstream changes that are required
• [ ] Added/updated documentation for these changes
• [ ] Added/updated tests for these changes
• [x] Verified changes in test deployment and summarized results, e.g. in PR description or comments on the related issue(s)

Reviewer checklist

• [ ] Have all dependencies been updated?
• [ ] Is the level of changes labeled appropriately?
• [ ] Are all the changes described appropriately in CHANGELOG.md?
• [ ] Has the documentation been adequately updated?
• [ ] Are the tests adequate?
• [ ] Have the changes been verified in the test deployment?