docs: add display compatibility, multiscreen, multiple VMs, Windows scripts, and iPhone passthrough
- Add display backend and VirGL acceleration compatibility table (#1448)
- Document max_outputs for multiscreen support (#518)
- Add guide for managing multiple VMs from the same OS image (#1282)
- Document Windows VM startup script customisation (#1665)
- Add experimental iPhone USB passthrough guide for macOS VMs (#468)
Resolves: #1665, #1448, #1282, #518
Partially addresses: #468