ci: sync README to Docker Hub description in publish job

[pyramation]

Summary

• The Docker Hub overview at hub.docker.com/r/constructiveio/postgres-plus is not synced from the repo README (it's a stale manual copy that still mentions pg_cron). Add a peter-evans/dockerhub-description@v4 step to the existing publish-postgres-plus-manifest job in docker.yml, pushing README.md and the short description to Docker Hub on main-branch publishes.
• Gated on the same DOCKERHUB_ENABLED (DOCKERHUB_USERNAME/DOCKERHUB_TOKEN secrets) as the image publish; also adds a checkout step to that job since it previously didn't need the repo contents.

Link to Devin session: https://app.devin.ai/sessions/f54f736b3cb24848bbb20474018bc261
Requested by: @pyramation

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