docs: document verify CLI usage and remove end-to-end section

[strimo378]

Motivation

• Update the README to reflect the CLI's new end-to-end verify command instead of describing a manual end-to-end workflow.
• Remove the stale CLI End-to-End Test Concept section which duplicated user-facing instructions already implemented in the tool.

Description

• Replace the compile --emit-testbench example with the verify usage example (python -m onnx2c verify path/to/model.onnx).
• Remove the old “CLI End-to-End Test Concept” paragraph and add a concise CLI Parameters section describing model, output, --template-dir, --model-name, and --emit-testbench.

Testing

• Ran the test suite with pytest -q and it succeeded with 21 passed.
• No additional automated tests were required for the README-only change.

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Codex Task