Document platform setup for ko

[greenmoon55]

Describe the change you'd like to see
Update Developer guide https://kserve.github.io/website/0.8/developer/developer/#deploy-kserve-from-master-branch
Users need to set up this env for M1 Macbooks export KO_DEFAULTPLATFORMS=linux/arm64

Additional context
Add any other context or screenshots about the feature request here.