docs: propose coordinated multi-pod snapshot restore

[julienmancuso]

Summary

• add SNEP-324 for coordinated multi-Pod checkpoint and restore
• define additive PodGroupSnapshot, PodGroupSnapshotContent, and
  PodGroupRestore APIs
• specify checkpoint, preflight, reconstruction, release, validation, failure,
  scheduling, monitoring, and qualification contracts

Tracking issue: #324

Validation

• make update-toc
• make verify-toc
• git diff --check

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