Add more resources in relevant domains

[dth99]

We welcome contributions to expand our curated list of DevOps labs, challenges, and projects! Please help us by adding high-quality, hands-on resources in any relevant domain (Linux, Kubernetes, Cloud, Security, CI/CD, Monitoring, etc.).

• Browse the README for existing resources and categories.
• Add your new link(s) with a one-line description under the appropriate heading.
• Make sure resources are hands-on, practical, and free.
• Open a pull request with your additions!

See CONTRIBUTING.md for details.

Thank you for making DevOps - Learn by Doing even better!