docs: make dartdoc examples self-contained

[miachillgood]

Closes #101.

Summary

• Wrap dartdoc examples that include imports in main() functions so they can be copied into runnable Dart files.
• Use Future<void> main() async for examples that await webcrypto APIs and void main() for the random-byte example.
• Fix a few keypair/wording typos in the touched examples while preserving the documented APIs.

Validation

• git diff --check
• Custom script verified all dartdoc dart code blocks with imports now include a main() function.