Add Proxmox VE server provider guide

[saeedvaziry]

Docs for the Proxmox VE server provider added in vitodeploy/vito#1259.

• New page 4.x/servers/proxmox (sidebar: Servers → Proxmox VE). An end-to-end guide:
  • requirements, including network reachability for SSH
  • creating the API token, with the Privilege Separation pitfall
  • building a cloud-init template from Ubuntu cloud images
  • DHCP vs static IP, and cluster notes
  • connecting in Vito and creating a server, with the plans table
  • how deletion works
  • troubleshooting keyed to the exact error messages Vito shows
• 4.x/settings/server-providers: lists Proxmox VE and its required roles, and links to the guide.
• 4.x/servers/create: notes the Proxmox-specific fields and links to the guide.

bun run build passes. On the built page, the anchors and links resolve, the tables render, and all callouts are converted.

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[saeedvaziry]

Closing: the docs live in the vito repo (docs/4.x), which the website syncs from. The Proxmox docs are in vitodeploy/vito#1259 instead.