Docs: switch order to match AWS IAM setup

What does this MR do?

When creating a new AWS access key with custom permissions, you have to create the permissions first, so I changed the order here of the AWS setup tasks.

Also "future-proofed" the section by adding an AWS header.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
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When applicable:

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