Updated Cheatsheets with New sources (#6191)

* Updated Cheatsheets with New sources

2 Links added.
1. C++ - Cheatsheet Greg Book 2002
2. MongoDB codecentric Cheatsheet

* Updated Commit - alphabetical order

* Updated and corrected space as per Guidelines

* Rollbacked incorrect Space Mistake

* Removed C++ cheatsheet, for which proper attribution source not available

* use correct title
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>
Co-authored-by: NEric Hellman <eric@hellman.net>
Co-authored-by: NDavid Ordás <3125580+davorpa@users.noreply.github.com>

more/free-programming-cheatsheets.md

@@ -173,6 +173,7 @@
 ### MongoDB
 
 * [MongoDB Cheat Sheet](https://www.mongodb.com/developer/quickstart/cheat-sheet) - MongoDB (HTML)
+* [MongoDB Cheat Sheet](https://blog.codecentric.de/files/2012/12/MongoDB-CheatSheet-v1_0.pdf) - codecentric (PDF)
 * [Quick Cheat Sheet for Mongo DB Shell commands](https://gist.github.com/michaeltreat/d3bdc989b54cff969df86484e091fd0c) - Michael Treat's Quick Cheat Sheet