MP user guide - add example for configuring AWS credentials in GHA

[lwilts]

The example linked in the user guide wasn't too helpful - I figured out how to assume the role by using this as an example instead

Of course it depends on the tooling being used, but it might be useful to mention this aws-actions/configure-aws-credentials action in the docs.

I was thrown initially by the fact the role assumption requires AssumeRoleWithWebIdentity; it could also be better explained how GHA authenticates to AWS.