add docs on configuring gpus

[eyberg]

https://github.com/nanovms/ops/pull/1391

[rinor]

there is this already https://docs.ops.city/ops/configuration#runconfig.gpus Is it enough or we want to mention smth also at https://docs.ops.city/ops/google_cloud

[eyberg]

I think the idea here was to extend ops functionality to integrate more of the work described here https://nanovms.com/dev/tutorials/gpu-accelerated-computing-nanos-unikernels - but that is going to require more thinking on how to best expose it (eg: the large volume w/the firmware https://www.nvidia.com/Download/driverResults.aspx/191320/en-us/ )