Docs: Add note around minimum compute requirements

[vdice]

With the current platform configuration seen in these examples, the minimum memory requirement appears to be 2GB to run both the platform and deployed app(s).

This came up when seeing if the platform could run on free tier instances, for example the t2.micro instance offered by AWS. This instance includes 1GB in memory which alas is nearly but not quite enough to support running both the platform services and additional deployed Spin apps, assuming all run with the default Nomad resource allotments at 300MB mem/100MHz CPU.

We should document this minimum requirement, along with minimum advised CPU. (For the same example as above, we were well under the t2.micro CPU limits, at 400 MHz out of a total 2.5 GHz, which included all platform services and one example app.)