Sprint 7: Checkout (Final)

What's Changed

• docs: plan Sprint 7 checkout in the PRD and add domain setup by @romulojr-dev in #97
• Email foundation and signed order links by @romulojr-dev in #98
• Order emails: placed, payment received, cancelled by @romulojr-dev in #99
• Vouchers at checkout by @romulojr-dev in #100
• Cash on delivery, confirmed by email by @romulojr-dev in #102

Full Changelog: v0.6.0...v0.7.0