Enhance VNC troubleshooting for black screen issues (#6673)

* Enhance VNC troubleshooting for black screen issues

Added troubleshooting steps for VNC black screen issue and clarified the importance of restarting WindowServer via SSH.

* Apply suggestions from code review

Co-authored-by: Firdevs Arslan <farslan@scaleway.com>

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Co-authored-by: Loic-kd <lkhamdy@scaleway.com>
Co-authored-by: Firdevs Arslan <farslan@scaleway.com>