docs: add AWS spot instance cost analysis for GPU training

[abrichr]

Summary

• Add comprehensive AWS spot instance pricing analysis for GPU training workloads
• Compare g5 (A10G) and g6 (L4) instances across single-GPU and multi-GPU configurations
• Include cost projections for 7-day training runs, interruption risk assessment, and actionable recommendations
• Document gotchas (EBS deletion, V100 incompatibility, VRAM limits) and implementation TODOs

Test plan

• Verify markdown renders correctly on GitHub
• Cross-reference pricing data with current AWS pricing page

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