docs(examples): webhook automation tutorials — maintenance migration and offline-aware load balancer

[guthrie-vast]

Summary

Two new client-facing examples under Examples → Notifications, plus a shared setup snippet. They show notification webhooks as an automation hook: each delivery carries a structured data object, so an event can drive an action instead of a chat message.

• examples/notifications/maintenance-migration.mdx — Migrate an Instance Before Scheduled Maintenance. On client:upcoming_downtime, look up the maintenance window, copy state off the instance, rent a replacement on a different machine, and destroy the original after instance_started.
• examples/notifications/offline-aware-load-balancer.mdx — Route Around Offline Instances with a Simple Load Balancer. On client:instance_offline (plural instance_ids[] / machine_ids[]), drain, blacklist the failed machine, rent a replacement elsewhere, and re-admit only after the client's own readiness probe.
• snippets/notifications/webhook-automation-setup.mdx — shared receiver (verify → dedupe → enqueue → 2xx), webhook creation, and a local event simulator so readers can test without waiting for real maintenance.
• docs.json — both pages added to the Notifications nav group.

Notes for review

• The upcoming_downtime payload currently carries instance_id, machine_id, maintenance_id but not the window timing. Tutorial 1 uses POST /api/v0/machines/maintenances (renter-callable, verified against the web backend) to fetch start_time / duration_hours. When the payload is extended to include those fields, the "Look Up the Maintenance Window" step becomes a fallback; the Note in that section is the spot to edit.
• instance_resumed fires on leaving Scheduling into running, stopped, or exited; Tutorial 2 checks actual_status before re-admitting.
• The load balancer is deliberately minimal (buffered, stdlib proxy) so the event handling is visible. It is framed as a model for the handlers, not a production proxy.
• All Python blocks parse; the handlers have not yet been run end to end against live instances. Draft until that is done.

Preview checklist

• Snippet import renders on both pages (<WebhookAutomationSetup />)
• Cross-links between the two pages and to the Slack example resolve
• Run both handlers against a real instance with the simulator

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