Add Official Matplotlib cheat sheets (#5468)

Add link to official Matplotlib.org cheat sheets github repo.

Resolve EbookFoundation/free-programming-books#5467

more/free-programming-cheatsheets.md

@@ -127,6 +127,7 @@
 * [Comprehensive Python Cheat Sheet for Beginners](https://medium.com/the-codehub/comprehensive-python-cheat-sheet-for-beginners-5d76bb038fa2)
 * [Comprehensive Python Cheatsheet](https://gto76.github.io/python-cheatsheet) - Jure Šorn (HTML)
 * [Learn Python in Y minutes](https://learnxinyminutes.com/docs/python/)
+* [Official Matplotlib cheat sheets](https://github.com/matplotlib/cheatsheets) - Matplotlib.org (LaTeX, PDF)
 * [Python Cheat Sheet](https://websitesetup.org/python-cheat-sheet/)
 * [Python Cheatsheet for beginners](https://www.codecademy.com/learn/learn-python-3/modules/learn-python3-syntax/cheatsheet) - Codeacademy (HTML)
 * [Python Crash Course Cheatsheet](https://ehmatthes.github.io/pcc/cheatsheets/README.html) - Eric Matthes (HTML)