Add Coding Challenges Newsletter

[JohnCrickett]

Description

Added the Coding Challenges Newsletter to the Additional Resources section

Motivation and Context

Adds another resource that provides weekly coding challenges to readers that they can use to improve their skills or learn a new programming language. Placed in the additional resources as it is language agnostic.

How Has This Been Tested?

Types of changes

• [x] Content Update (change which fixes an issue or updates an already existing submission)
• [ ] New Article (change which adds functionality)
• [ ] Documentation change

Checklist:

• [ ] My code follows the code style of this project.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [x] I have made checks to ensure URLs and other resources are valid