docs: add benchmark snapshot to README

[AyushmanRaha]

Motivation

• Provide a concise, local ONNX vs PyTorch benchmark snapshot adjacent to the ONNX guidance so readers have concrete performance context while preserving that ONNX is optional.

Description

• Inserted a Benchmark snapshot section and matching TOC entry immediately after ONNX acceleration note containing the provided table for MiDaS Small, DPT Hybrid, and DPT Large and a professional note clarifying this is a local snapshot, that standard builds do not require ONNX (PyTorch is the default), and that DPT Hybrid/Large ONNX runs were skipped on this fanless MacBook Air to avoid sustained heat, lag, and memory pressure.

Testing

• Ran python scripts/validate_docs.py and scripts/ci.sh docs-contract, and both checks passed.

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Codex Task