Document usage of AWS env vars

[nadilas]

Description

Document the working method of using environment variables on the external-dns deployment to avoid the overhead of kiam or kube2iam when not using EKS

Fixes undocumented feature

Checklist

• Unit tests updated
• End user documentation updated
• AWS tutorial updated

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/lifecycle stale

[nadilas]

/remove-lifecycle stale

[Raffo]

/approve
/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nadilas, Raffo

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