CI: Add write permissions, per docs

[parasyte]

• From https://github.com/peaceiris/actions-gh-pages?tab=readme-ov-file#%EF%B8%8F-first-deployment-with-github_token
• See also: https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#permissions

This is an attempt to fix the deployment failure. We won't know if it works until after this is committed to the main branch. :(