docs: add cloud-init / ubuntu sample for RPi-4

[shantanoo-desai]

• add sample user-data file cloud-init setup
• ignore generated images directory
• add Packer template ubuntu.pkr.hcl
• complete documentation of usage for cloud-init / packer

Closes #148

Signed-off-by: Shantanoo 'Shan' Desai shantanoo.desai@gmail.com

[solo-build-bot[bot]]

Waiting for approval from someone in the solo-io org to start testing.