Add docs examples for "Add local file embedding for SSH keys and systemd units"

[travier]

See https://github.com/coreos/butane/pull/289

[Okeanos]

During my work on the PR I created some local test files I can reuse. They are currently on a branch (I'll need to and will update) in my fork of this example repo and can be seen here.

I'll see about creating a documentation PR for that.

The PR should probably target the following sections of the tutorials I guess:

• Setting up SSH access and starting containers at boot
  • Possibly also Configuring Users
• Starting a script on first boot via a systemd service
  • Possibly also Running Containers
• Managing Files, Directories and Links

If there's anything else you envision or a better placement for this please let me know.