Add Kubernetes troubleshooting page to nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab-runner!4622 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:
• Links:
  • https://fneill-add-k8s-troubleshooting-to-nav.docs.gitlab-review.app/runner/executors/kubernetes/
  • https://fneill-add-k8s-troubleshooting-to-nav.docs.gitlab-review.app/runner/executors/kubernetes/troubleshooting.html