feat: Add CISA Secure by Design cheat sheet and update Secure Product Design sheet #1793

[Prasad-JB]

Hi team,

This pull request addresses issue #1793 by updating our Secure Product Design guidance to align with the modern CISA Secure by Design principles.

What's in this update?

I've made two main changes:

1. Created a brand new cheat sheet: cisa_secure_by_design_principles_cheat_sheet.md
   This is a dedicated guide that breaks down each of the 7 CISA principles in plain language, explaining what they mean and how to put them into practice.

2. Gave the existing Secure Product Design cheat sheet a refresh:

   • Added a note at the top to connect it to the CISA principles.
   • Fixed the broken links to the CISA resources (the old ones were 404s).
   • Added a handy new "Security Definitions" section to clear up common confusions (like encoding vs. escaping).
   • Kept all the great original content intact, this is just a modernization.

Why this is useful

CISA's principles are quickly becoming the industry standard for building secure software. By bringing our cheat sheets in line with them, we make our guidance more immediately useful for developers and architects who are following this modern approach. It helps shift the focus from bolting on security to building it in from the start.

This should close out the issue. Thanks for the review!

Closes #1793

[szh]

Closing as per #1799 (comment)