Deploy docs CI is broken due to missing secrets in GitHub Actions

[git-hulk]

Seems ACTIONS_DEPLOY_KEY was removed from secrets a few months ago, and it caused the CI failure while deploying the documentation. I'm not sure if it's an expected operation. Perhaps we can replace the deploy key with GITHUB TOKEN?

CI error: https://github.com/cloudspannerecosystem/memefish/actions/runs/10472793495/job/29003115616