docs: add OIDC-based AWS authentication example for ECR scanning

[doyindevops]

The current AWS ECR section uses static AWS credentials
(AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY). This is discouraged
by AWS as it involves long-lived credentials that increase security risk.

This PR adds an alternative example using GitHub Actions OIDC with
aws-actions/configure-aws-credentials@v4 — the current AWS-recommended
approach. No static credentials required; authentication happens via
short-lived tokens tied to the workflow.

This pattern is commonly used in production EKS environments and
aligns with AWS security best practices.

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