WIP: docs: move HMAC importRawKey example to file

[Apple-beep]

Addresses part of #283

Summary

• Moves the HmacSecretKey.importRawKey dartdoc example into a standalone file under example/webcrypto/hmac/import_raw_key.dart.
• Replaces the inline dartdoc code block with a {@example ...} reference.
• Keeps this scoped to one HMAC example as an initial primitive-level cleanup.

Notes

• Opened based on maintainer guidance in Move dartdoc examples to separate files #283.
• This PR builds on chore: ignore unknown dartdoc example directive #294, which ignores doc_directive_unknown until analyzer support for {@example ...} lands.
• This PR intentionally does not use Fixes #283 because it only addresses part of the larger issue.

Testing

• dart format lib/src/webcrypto/webcrypto.hmac.dart example/webcrypto/hmac/import_raw_key.dart
• dart analyze

[Apple-beep]

Hi @jonasfj, I rebased this PR after #294 was merged.

It looks like the workflows are still awaiting maintainer approval. Could you please approve the workflow run when you get a chance?

Thanks!