Configure Renovate to rebase PRs less frequently

[jattasNI]

Pull Request

🤨 Rationale

Once Renovate has created PRs it currently updates them any time something else changes in the repo. This is annoying because it uses lots of build resources and makes it hard to track down a completed build for the PR, for example to inspect Storybook.

In a PR discussion we decided to switch to manually triggering the rebase before merging the PR.

👩‍💻 Implementation

1. Update renovate.json to rebaseWhen: conflicts. This seems more helpful than never but hopefully less noisy than auto.
2. Update CONTRIBUTING to mention that owners may need to manually trigger the rebase

🧪 Testing

We'll see if it works the next time Renovate creates PRs.

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.