docs(amazon-bedrock): add API key authentication documentation

[zapagenrevdale]

Background

The Amazon Bedrock provider recently added support for API key authentication via the AWS_BEARER_TOKEN_BEDROCK environment variable and apiKey parameter.
However, the documentation was missing this information, and the package README contained incorrect references to a non-existent withSettings() method.

Summary

• Added API key authentication documentation as the recommended method for Amazon Bedrock
• Fixed incorrect bedrock.withSettings() references to use createAmazonBedrock() instead
• Added apiKey parameter to provider settings documentation
• Documented authentication precedence: API key → environment variable → SigV4

Manual Verification

N/A - Documentation-only changes. Verified against implementation in packages/amazon-bedrock/src/bedrock-provider.ts.

Checklist

• Tests have been added / updated (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)
• A patch changeset for relevant packages has been added (for bug fixes / features - run pnpm changeset in the project root)
• Formatting issues have been fixed (run pnpm prettier-fix in the project root)
• I have reviewed this pull request (self-review)

Related Issues

N/A