ci: run prettier before opening docs PR

[kenjenkins]

Summary

There is a GitHub Actions workflow to generate the docs page https://www.pomerium.com/docs/k8s/reference. This workflow is currently opening spurious PRs due to formatting differences. If we run prettier first we should be able to match the existing formatting in the documentation repo, which will hopefully avoid this issue.

Related issues

• https://github.com/pomerium/ingress-controller/issues/695

Checklist

• [x] reference any related issues
• [ ] updated docs
• [ ] updated unit tests
• [ ] updated UPGRADING.md
• [x] add appropriate tag (improvement / bug / etc)
• [ ] ready for review