AWS prescriptive-guidance medium security documentation change
Summary
Renamed file from understanding-landing-zones.md, updated content structure, added security isolation bullet points with 'blast radius' terminology, and included architecture diagram
Security assessment
Introduces security-focused language by changing 'scope of impact' to 'blast radius' (a security operations term) and consolidates security isolation considerations. The explicit listing of security isolation requirements (administrative isolation, limited visibility, audit data protection) enhances security documentation.