Add a user-facing Troubleshooting page

Description

The guide has no page a user can turn to when something goes wrong. The Virtualization Debugging section is written for developers who need to attach gdb or strace to QEMU, and the troubleshooting notes that do exist for users are embedded in unrelated pages such as Accessing Virtual Machines and Memory Dump.

Add a Troubleshooting page, separate from Virtualization Debugging, that lists common symptoms with likely causes and links to the fix. Start with these symptoms:

• A VirtualMachineInstance stuck in Scheduling or Pending.
• A failed live migration.
• Console or VNC connection errors.
• Missing qemu-guest-agent data.

For each symptom, give the kubectl or virtctl command that reveals the cause, the two or three most likely causes, and a link to the page that explains the fix. Move the troubleshooting notes from Accessing Virtual Machines and Memory Dump onto this page, or link to them from it. Add the page to docs/.nav.yml near Virtualization Debugging and link to it from the Getting Started page.

What you expected

A user who hits a common failure can find a symptom-oriented page in the guide that leads them to the cause and the fix without reading the developer debugging section.

URL

• https://kubevirt.io/user-guide/debug_virt_stack/debug/ (docs/debug_virt_stack/debug.md)
• https://kubevirt.io/user-guide/user_workloads/accessing_virtual_machines/ (docs/user_workloads/accessing_virtual_machines.md)
• https://kubevirt.io/user-guide/compute/memory_dump/ (docs/compute/memory_dump.md)

Additional context

Keep the first version small. Four symptoms with accurate causes are more useful than a long list; the page can grow as users report more.