docs: clarify nitro cloudflare-durable preset in WebSocket setup

[michaelreinhard1]

Fix Realtime & WebSockets docs by adding required cloudflare-durable preset. The hub.workers TypeScript hint says it should automatically set the preset to cloudflare-durable when nitro nitro.experimental.websocket is enabled, but this doesn't actually happen during build.

[vercel[bot]]

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[atinux]

Sorry @michaelreinhard1 I am closing it now as we migrated to multi-vendor, please feel free to open a new one on the new docs