Adding CSS Cheat Sheet (#8020)

This is a very helpful cheat-sheet provided by Stanford University.

more/free-programming-cheatsheets.md

@@ -163,6 +163,7 @@
 * [Accessibility CheatSheet](https://learn-the-web.algonquindesign.ca/topics/accessibility-cheat-sheet/) - Algonquin Design (HTML)
 * [CSS CheatSheet](https://htmlcheatsheet.com/css/) (HTML)
 * [CSS Cheatsheet](https://www.codewithharry.com/blogpost/css-cheatsheet/) - CodeWithHarry (HTML)
+* [CSS Cheatsheet](https://web.stanford.edu/group/csp/cs21/csscheatsheet.pdf) - Stanford (PDF)
 * [CSS Flexbox Cheatsheet](https://css-tricks.com/snippets/css/a-guide-to-flexbox/) - Chris Coyier (HTML)
 * [CSS Grid Cheatsheet](https://css-tricks.com/snippets/css/complete-guide-grid/) - Chris House (HTML)
 * [FLEX: A simple visual cheatsheet for flexbox](https://flexbox.malven.co) - Chris Malven (HTML)