docs: align README with actual default attestation types and pipeline

[kazuhidelee]

Summary

This PR updates the README so it matches the current CLI behavior.

Changes

• added network-trace to the documented default attestation types
• clarified the processing pipeline so it better reflects the current generation flow

The CLI currently defaults to:

• material
• command-run
• product
• network-trace

currently the README only mentioned the first three.

References

• cmd/generate.go#L59
• README.md#L37