deploy docs only on push to master

[petschki]

The deploy step also ran on pull_request events and published the PR build to gh_docs_deploy. Since the branch got protected, the push is rejected and every PR fails the docs job. Skip the deploy for PR builds; they only need to verify that the docs build.

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't any other open pull requests for the same change.
• I followed the guidelines in Contributing to Plone.
• I successfully ran code quality checks on my changes locally.
• I successfully ran tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes.
• I included a change log entry in my commits.

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