docs(cheatsheets): align Secure Product Design with verified CISA Secure by Design principles

[Prasad-JB]

This PR manually adds verified CISA Secure by Design principles (architecture, memory safety, developer culture, and secure defaults) to the Secure Product Design Cheat Sheet.
Fixes: #123
Notes:

• All content is manually written and verified from official CISA sources.
• Fixed broken reference link (/secure-design → /securebydesign).
• Verified lint checks and formatting compliance.

[mackowski]

This is a good start but I would try to incorporate CISA Secure by Design principles with the document, not create a separate section.