docs: explain CPU and GPU attestation

[h4x3rotab]

Summary

• explain how application verifiability is tied to CPU TEE attestation
• distinguish application and CPU evidence from GPU environment evidence
• clarify the boot-time trust flow and all-GPUs-must-pass behavior for multi-GPU deployments
• connect the conceptual overview to remote verification and the detailed security model

Testing

• git diff --check origin/next...HEAD
• prek run trailing-whitespace end-of-file-fixer --from-ref origin/next --to-ref HEAD