Update minimum boot disk size in GPU tutorial

[95-martin-orion]

In the GPU-on-cloud tutorial, we advertise that a boot disk size of 30GB is the minimum necessary for installing everything. This is no longer the case; users have reported overloading a disk of this size during the install step, which is a bad experience as it tends to happen very late in the process.

The same users report that 100GB is sufficient. We should attempt to tighten the bound here, but until that happens we should use that as the doc value to prevent a poor user experience.