ci: doc-publish: Do not run for pull requests

GitHub Actions may trigger the `workflow_run` event type when the
pull request source branch name matches one of the branches specified
under `branches:`.

This commit updates the documentation publish workflow to skip when the
preceding event type is `pull_request`, in order to prevent pull
request documentation from being uploaded as main documentation.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>