Fix Markdown Lint Issues in Secure Product Design Cheat Sheet

[Prasad-JB]

This PR fixes all Markdown linting issues in the Secure Product Design Cheat Sheet.

Changes include:

• Adding blank lines around headings and lists.
• Ensuring proper formatting for readability.
• Updating sections to comply with markdownlint rules.

Purpose:

• Make the cheat sheet cleaner and easier to read.
• Ensure it passes the repository's Markdown lint checks.
• Keep the focus only on Secure Product Design, without affecting other cheat sheets.
  This is a standalone fix and does not touch any other files.

[mackowski]

I am closing this as it makes changes to many cheatsheets in one PR