Add DOIs to citations on RTD

[trey-stafford]

Description

Add DOIs to QGreenland citations on ReadTheDocs. https://qgreenland.readthedocs.io/en/latest/citing.html

Checklist

If an item on this list is done or not needed, check it with [x] or click the checkbox.

• [x] The PR description links to issues that it resolves with closes #{issue_number}
• [ ] Config lockfile updated (inv config.export > qgreenland/config/cfg-lock.json)
• [x] Environment lockfile updated if needed (conda-lock)
• [x] Version bumped if needed (bumpversion (major|minor|patch|prerelease|build))
• [x] CHANGELOG.md updated (for user-facing changes)
• [x] Documentation updated if needed
• [x] New unit tests if needed