#1079 Enable preview URLs for docs Worker versions

Summary

The Docs preview: sticky comment on PRs has been empty (e.g. #1077, #1078): the workflow interpolates steps.deploy.outputs.deployment-url, but wrangler versions upload only prints the Version Preview URL line that populates the output when preview URLs are enabled for the Worker. This adds preview_urls = true to wrangler.toml.

Note: preview URLs are served on workers.dev, so the account's workers.dev subdomain must be registered (one-time dashboard setting) for wrangler to print the URL.

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.