docs: extract AES-GCM importRawKey example

[agustinpabon]

Summary

Extract the existing AesGcmSecretKey.importRawKey example into
example/webcrypto/aes_gcm/import_raw_key.dart and reference its
example region from the API documentation.

This makes the example available to formatting, analysis, and execution
tools while preserving its behavior and leaving the library
implementation unchanged.

Part of #283.

The new file follows the existing example structure, including the
license header and region markers. A file-local avoid_print
suppression preserves the example's demonstrated output.

Validation

Recorded locally on Windows x64 with Dart 3.13.3:

• Formatting checks passed for the changed Dart files.
• Full-package analysis completed without diagnostics.
• Native execution of the extracted example printed hello world.
• The focused AES-GCM test selection passed all 82 tests.
• The method's rendered description HTML was unchanged before and after extraction: no documentation errors and the same seven pre-existing warnings as the baseline.
• Extraction comparison confirmed that the example code was preserved.

The full platform matrix was not run.

AI assistance

OpenAI Codex assisted with extraction and validation. The contributor
personally reviewed the diff and generated API documentation before
submission.

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