docs(cloud): align AWS role mode and external ID steps with UI

[llewellyn-sl]

Summary

• align AWS credential instructions with current UI behavior for both Cloud and Enterprise (unversioned docs)
• clarify Keys vs Role mode behavior and role ARN usage in Assume role
• document mode-specific External ID behavior:
  • Keys mode: Generate External ID appears only when an ARN is provided in Assume role, and generation is optional
  • Role mode: External ID is generated automatically on save
• improve readability of credential setup with clearer mode-specific substeps

Files changed

• platform-cloud/docs/compute-envs/aws-batch.md
• platform-cloud/docs/compute-envs/aws-cloud.md
• platform-cloud/docs/compute-envs/eks.md
• platform-enterprise_docs/compute-envs/aws-batch.md
• platform-enterprise_docs/compute-envs/aws-cloud.md
• platform-enterprise_docs/compute-envs/eks.md

Notes

• Cloud instructions reference arn:aws:iam::161471496260:role/SeqeraPlatformCloudAccessRole where applicable.
• Enterprise instructions use deployment-configured IAM role ARN guidance and retain Enterprise-specific caveats.

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