Adding free courses (#7640)

Added the following courses 
- Blockchain Essentials
- Docker Essentials: A Developer Introduction
- IBM Cloud Essentials V3
- Introduction to Cloud

All from CognitiveClass.ai

courses/free-courses-en.md

@@ -229,6 +229,7 @@
 ### Blockchain
 
 * [Blockchain Developer Tutorials](https://www.youtube.com/playlist?list=PLS5SEs8ZftgUTXs0OJD2LFpYBPr4L54id) - Gregory McCubbin (Dapp University)
+* [Blockchain Essentials](https://cognitiveclass.ai/courses/blockchain-course) - CognitiveClass.ai
 * [Blockchain: Foundations and Use Cases](https://www.coursera.org/learn/blockchain-foundations-and-use-cases) - ConsenSys Academy (Coursera)
 * [Blockchain Specialization](https://www.coursera.org/specializations/blockchain) - Bina Ramamurthy (Coursera)
 * [Build 5 Dapps on the Ethereum Blockchain - Beginner Tutorial](https://www.youtube.com/watch?v=8wMKq7HvbKw) - Julien Klepatch, EatTheBlocks (freeCodeCamp.org)
@@ -280,6 +281,8 @@
 
 * [Cloud Computing and Development](https://uaceit.com/courses/cloud-computing-and-development) - Ashwin Kumar Ramaswamy (UAceIt) (email address *required*)
 * [Cloud Computing Full Course In 11 Hours \| Cloud Computing Tutorial For Beginners](https://www.youtube.com/watch?v=2LaAJq1lB1Q) - Edureka!
+* [IBM Cloud Essentials V3](https://cognitiveclass.ai/courses/ibm-cloud-essentials) - CognitiveClass.ai
+* [Introduction to Cloud](https://cognitiveclass.ai/courses/introduction-to-cloud) - CognitiveClass.ai
 * [Microsoft Certified: Azure Fundamentals](https://docs.microsoft.com/en-us/learn/certifications/azure-fundamentals/) - Microsoft
 
 
@@ -416,6 +419,7 @@
 ### Docker
 
 * [Docker](https://www.youtube.com/playlist?list=PLhW3qG5bs-L99pQsZ74f-LC-tOEsBp2rK) - Raghav Pal
+* [Docker Essentials](https://cognitiveclass.ai/courses/docker-essentials) - CognitiveClass.ai
 * [Docker Tutorial for Beginners](https://www.youtube.com/playlist?list=PL6gx4Cwl9DGBkvpSIgwchk0glHLz7CQ-7) - Thenewboston
 * [Docker Tutorial for Beginners](https://www.youtube.com/watch?v=fqMOX6JJhGo) - FreeCodeCamp
 * [Docker Tutorial for Beginners \[FULL COURSE in 3 Hours\]](https://www.youtube.com/watch?v=3c-iBn73dDE) - TechWorld with Nana