Update `oxide_instance` documentation to show valid cloud-init usage

[sudomateo]

This is related to my stance that the example configurations in the documentation should be usable with minimal changes by the user. We cannot assume that everyone coming to Oxide is familiar with the concepts and functionality it provides.

For example, our current oxide_instance documentation has the following.

resource "oxide_instance" "example" {
  # ...
  user_data = filebase64("path/to/init.sh")
}

We can do better than that and give the user a simple cloud-init configuration that creates a user or something. Let's do that!

[linear]

SSE-186