docs: Cloudflare deployment guide (SSR + static + Workers)

[vertz-tech-lead]

Summary

Create documentation covering how to deploy vertz apps to Cloudflare, including:

Guides Needed

1. Deploy a vertz SSR app to Cloudflare Workers — using @vertz/ui-server's renderToStream() in a Worker fetch handler
2. Deploy a static vertz app to Cloudflare Pages — Vite build + Pages deployment
3. Preview deployments on PRs — GitHub Action setup with wrangler-action
4. Custom domains — pointing your domain at Cloudflare Workers/Pages

Content Should Cover

• wrangler.toml configuration for vertz apps
• Worker entry point pattern (wrapping renderToStream in a fetch handler)
• Vite SSR build configuration (@vertz/ui-compiler with ssr: true)
• Environment variables and secrets management
• Preview vs production deployment
• GitHub Action workflow template (reusable across examples)

Context

We're deploying the task manager as the first example (issue #377). This documentation should generalize the patterns we establish there so all future examples and demos can follow the same approach.

Location

These docs go in backstage/packages/docs/ (Mintlify) under a 'Deployment' section.

Relates to #377