docs: document virtual machine management through the dashboard

[IvanHunters]

Part of #605.

virtualization/vm-instance.md / vm-disk.md are CLI/YAML-only (autogenerated). Cozystack 1.4 added WebSocket VNC to the dashboard; there is no doc for managing VMs in the UI.

Dashboard steps to document: deploy a VMInstance and a VMDisk from the marketplace, open the VNC console, and perform lifecycle actions (start/stop, attach disk).

Screenshots to capture: VM deploy form, VM detail page, the VNC console window.