Document how to set up a Raspberry Pi cluster for Ballista

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I am in the process of setting up a Raspberry Pi cluster as one of my benchmark/QA environments for Ballista and I will document the process as I go. This can be published as part of the user guide perhaps.

I am using the Ubuntu for Raspberry Pi operating system and microk8s.

Describe the solution you'd like
Comprehensive documentation.

Describe alternatives you've considered
None

Additional context
None