Add a top-level Deployment docs page

Deployment is the core workflow of Miren, but there's no dedicated docs page for it. We should add a top-level "Deployment" page under Features that covers:

• miren deploy basics (from a project directory, from a git repo, etc.)
• Build detection and configuration
• deploy --version for redeploying existing versions
• miren rollback for interactive rollback
• Deployment history (miren app history)
• deploy cancel for stuck deployments
• Deploy-time env vars (-e, -s)
• deploy --analyze for previewing detection
• Deployment provenance tracking

This came up while writing the changelog — we wanted to link "first-class rollbacks" to a docs page and realized there's nothing to link to.