docs: Add CVE rollout blog and tidy up member cluster guide.

[Simon Waight (sjwaight)]

This pull request updates and expands the KubeFleet documentation and blog with a focus on improving clarity, accuracy, and guidance for managing member clusters and mitigating kernel LPE CVEs. The main changes include a new blog post on CVE mitigation rollouts and substantial improvements to the member cluster lifecycle documentation, making instructions clearer and more consistent with KubeFleet terminology and workflows.

New Blog Post: Kernel LPE CVE Mitigation

• Added a comprehensive guide on using KubeFleet to roll out mitigations for several kernel LPE CVEs (CVE-2026-31431, CVE-2026-43284, CVE-2026-43500, CVE-2026-46300) across multiple Kubernetes clusters, including step-by-step instructions for using DaemonSets and KubeFleet's staged rollout features.

Documentation Improvements: Member Cluster Lifecycle

• Rewrote and clarified the process for adding and removing clusters from a fleet, emphasizing KubeFleet terminology (e.g., "member cluster" instead of just "cluster"), and updated instructions for both automated scripts and manual steps.
• Updated manual joining instructions to use consistent variable names (e.g., MEMBER_CLUSTER_NAME), clarified service account creation, and improved explanations for security and permissions. [1] [2]
• Revised the manual installation process for the member agent to use the latest KubeFleet images from the OCI registry, updated Helm install commands, and removed legacy references to Azure Fleet. [1] [2]
• Improved instructions for removing a cluster from a fleet, clarifying the cleanup process and how to uninstall the member agent.