index.md and rationale.md both mention this, but getting-started.md
currently doesn't, except when discussing systemd services in the
troubleshooting section. Add a mention and reference for anyone who
starts with the getting-started guide.
Alternative to https://github.com/coreos/ignition/pull/1206.
The text `first boot` currently links to the top of the systemd documentation on condition directives by looking to `ConditionArchitecture`, which is confusing. I think it would be clearer to link to `ConditionFirstBoot` directly given the text and context of the link.
