Added Datastax Academy (#6329)

* Added Datastax Academy

Added Datastax Academy to the list of NoSQL courses.

* Changed list to follow alphabetical order.

* Changed link to Datastax Academy

Changed link to Datastax Academy so authentication is not required

courses/free-courses-en.md

@@ -284,6 +284,7 @@
 
 #### NoSQL
 
+* [Datastax Academy (Apache Cassandra)](https://www.datastax.com/dev/academy) - Datastax Inc. *(email address required)*
 * [MongoDB University](https://university.mongodb.com) - MongoDB, Inc. (email address *required*)
 * [Neo4j Graph Database Tutorial](https://www.youtube.com/playlist?list=PLqfPEK2RTgChcOZ6qHgSfwiBPCz2Bzdjh) - Satish C J (YouTube)
 * [Redis University](https://university.redis.com) - Redis Inc. *(email address required)*