ci: switch back to yarn for docs repo

[kenjenkins]

Summary

The documentation repo is still using yarn, so the npm command to run prettier was not working as intended. As a result the script generated a number of formatting-only PRs in the documentation repo. Switch back to yarn for now, until we update the documentation repo to use npm.

Error message from a recent workflow run:

npm error code EUSAGE
npm error
npm error The `npm ci` command can only install with an existing package-lock.json or
npm error npm-shrinkwrap.json with lockfileVersion >= 1. Run an install with npm@5 or
npm error later to generate a package-lock.json file, then try again.

Related issues

https://linear.app/pomerium/issue/ENG-3286/docs-ingress-controller-automated-docs-prs-are-not-formatting

Checklist

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