Document Kubernetes CI

What does this MR do?

This change adds comprehensive documentation for how GitLab Runner's Kubernetes integration tests work. The new document explains that these tests verify GitLab Runner functions correctly with Kubernetes clusters by running against a dedicated test cluster in Google Cloud Platform.

The documentation covers the complete testing workflow: how test infrastructure is set up using a blue-green deployment model, how tests are organized into different categories (standard tests, legacy compatibility tests, and newer attach-strategy tests), and how security permissions are automatically generated based on the actual code being tested.

Key aspects documented include the three-stage pipeline process (provision resources, run parallel tests, cleanup), the use of unique service accounts for each test run to prevent conflicts, automated resource cleanup to maintain cluster health, and comprehensive monitoring through dashboards and logs.

The document also provides troubleshooting guidance for common issues like test timeouts, permission problems, and resource conflicts, along with debugging steps and commands. It explains that while tests are designed for the CI environment, local testing is possible with proper cluster access and setup.

This documentation serves as a complete reference for developers and operators who need to understand, maintain, or troubleshoot the Kubernetes integration testing system.

Why was this MR needed?

What's the best way to test this MR?

What are the relevant issue numbers?

Closes #38970 (closed)