Remove deprecated and redundant Read the Docs pull request preview workflow

[stevepiercy]

See https://community.plone.org/t/remove-rtd-pr-preview-build-workflow-file-and-a-warning-about-github-actions-v7/23083

• 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 other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I successfully ran code linting checks on my changes locally.
• I successfully ran unit tests on my changes locally.
• I successfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

[read-the-docs-community]

Documentation build overview

📚 volto | 🛠️ Build #33504450 | 📁 Comparing 5bff465 against latest (a888abc)

  🔍 Preview build  

1 file changed

± release-notes/index.html