docs: add cardinal-infrastructure stack runbook (#92)

Step-by-step runbook for the standalone cardinal-infrastructure
CloudFormation stack, covering both brand-new creation and adopting
an existing data-setup.sh-created install via CloudFormation resource
import. Spells out the always-required parameters, the import-only
name/identifier parameters (and why they must stay blank for fresh
creates and unchanged after import), and the optional tuning and
recovery-only overrides.

Author: skandragon