docs: expand artifact getter options
docs: expand artifact getter options

Adds an example of using HTTP Basic Auth, git options, and using HCL2 syntax
to encode an SSH key from file.

hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad