docs: move Hash.digestBytes example to a separate file

[ting-hong-shieh]

Summary

• move the existing Hash.digestBytes example to example/webcrypto/hash/digest_bytes.dart
• reference the extracted example from the API documentation with {@example /example/webcrypto/hash/digest_bytes.dart#example}
• keep the example behavior unchanged; add only the license header, region markers, and the avoid_print suppression required after extraction

This makes the example visible to formatting and analysis tools as requested in #283.

Validation

• dart format --output=none --set-exit-if-changed . — 122 files checked, 0 changed
• dart analyze — no issues found
• dart compile js example/webcrypto/hash/digest_bytes.dart -o /tmp/digest_bytes.js — succeeded

Native execution was not run because the Dart container does not include CMake, which the package's native build hook requires.

Towards #283.

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