Removed the docplayer platform (#5616)

* Update free-courses-en.md

* Added 2 open online courses and a book for ML

Added two open online courses in MOOC and added a book for machine learning

* Remove trailing slash 

Remove trailing slash

* Alphabetical ordering of courses

* Removed extra space

* Removed the ML book

courses/free-courses-en.md

@@ -73,10 +73,12 @@
 
 ### 0 - MOOC
 
+* [Codecademy](https://www.codecademy.com)
 * [Coursera](https://www.coursera.org)
 * [edX](https://www.edx.org)
 * [freeCodeCamp](https://www.freecodecamp.org)
 * [FutureLearn](https://www.futurelearn.com)
+* [Khan Academy](https://www.khanacademy.org)
 * [MIT OCW](http://ocw.mit.edu)
 * [MOOC.fi](https://www.mooc.fi/en/)
 * [NPTEL](https://onlinecourses.nptel.ac.in)