Docs migration from XPK to CTK(4/5)

[KevinYenky]

Description

This PR is the fourth part of decomposing the XPK to Cluster Toolkit document migration. It focuses on updating our documentation regarding Checkpointing Solutions to use Cluster Toolkit (CTK) instead of the deprecated XPK launcher.

Specifically, this PR updates the following guides:

• Multi-Tier Checkpointing: Replaces XPK-specific deployment and recovery commands with their gcluster equivalents, updating environment variables and volume mount instructions as necessary.
• Emergency Checkpointing: Updates the workflow and script references for handling emergency saves and preemptions to align with CTK's job lifecycle management.

Why this is being made:
CTK is replacing XPK as the primary submission mechanism for Google Kubernetes Engine (GKE) workloads. Breaking down the massive PR #5175 into smaller, logical chunks makes the migration significantly easier to review and test.

Tests

Tested by scripts that run the gcluster commands provided in the documents, with results stored in a GCS bucket.

Checklist

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents.

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[gemini-code-assist]

Code Review

This pull request updates the documentation for emergency and multi-tier checkpointing to transition from XPK-based instructions to Cluster Toolkit (gcluster) workflows. It includes detailed steps for configuring GKE clusters, setting up Cloud Storage buckets with Hierarchical Namespace (HNS), granting necessary IAM permissions, and submitting workloads. The review comments correctly identify that the documented commands for setting gcluster configuration defaults are incorrect and should be updated from gcluster job config set to gcluster config set to prevent command execution errors.