Add Kubernetes Cluster Playground Links

[pmady]

Description

Add links and instructions for free Kubernetes playground environments for hands-on practice.

Requirements

• Links to free playground environments (Killercoda, Play with Kubernetes, etc.)
• Setup instructions for local environments (kind, minikube, k3s)
• Cloud provider free tier instructions (GKE, EKS, AKS)
• Pre-configured scenarios for each certification

Acceptance Criteria

• Dedicated page for playground resources
• Step-by-step setup guides
• Comparison table of different options
• Links to pre-built scenarios