Write doc for Virtual Desktop OnDemand App

[jen-reeve]

The Virtual Desktop doc needs to be written.

[geoffreyweal]

What needs to be written about it, what does it need to cover?

[jen-reeve]

I think when I made this it was the only OOD app without any docs. No idea what to write, hence not actually doing it. maybe @CallumWalley has ideas?

[geoffreyweal]

done: #1320